DEPARTMENT OF THE ARMY TECHNICAL MANUAL · tm 9-258 technical manual headquarters department of the army no. 9-258 washington, dc, 5 december 1977 elementary optics and application

TM 9-258

DEPARTMENT OF THE ARMY TECHNICAL MANUAL

ELEMENTARY OPTICS

AND

APPLICATION TO

FIRE CONTROL INSTRUMENTS

HEADQUARTERS, DEPARTMENT OF THE ARMY

DECEMBER 1977

TM 9-258

TECHNICAL MANUAL HEADQUARTERSDEPARTMENT OF THE ARMY

No. 9-258 WASHINGTON, DC, 5 December 1977

ELEMENTARY OPTICS

AND

APPLICATION TO FIRE CONTROL INSTRUMENTS

Reporting Errors and Recommending Improvements. You can help improve this manual. If you find anymistake or if you know of a way to improve the procedures, please let us know. Mail your letter, DA Form2028 (Recommended Changes to Publication and Blank Forms), or DA Form 2028-2 located in the backof this manual direct to: Commander, US Army Armament Materiel Readiness Command, ATTN:DRSAR-MAS, Rock Island, IL 61201. A reply will be furnished

Paragraph PageCHAPTER 1. INTRODUCTION .................................................................................................. 1-1 1-1

CHAPTER 2. PROPERTIES OF LIGHTSection I. Light ...................................................................................................................... 2-1 2-1

II. Reflection .............................................................................................................. 2-5 2-9III. Refraction.............................................................................................................. 2-10 2-15IV. Image formation .................................................................................................... 2-18 2-26V. Color...................................................................................................................... 2-26 2-37VI. Characteristics of optical systems......................................................................... 2-29 2-39VII. Aberrations and other optical defects ................................................................... 2-37 2-42

CHAPTER 3. THE HUMAN EYESection I. General ................................................................................................................. 3-1 3-1

II. Construction .......................................................................................................... 3-3 3-2III. Binocular (two-eyed) vision and stereovision........................................................ 3-7 3-7IV. Defects and limitations of vision............................................................................ 3-10 3-14V. Optical instruments and the eyes.......................................................................... 3-12 3-17

CHAPTER 4. OPTICAL COMPONENTS, COATED OPTICS, AND CONSTRUCTION FEATURESSection I. Optical components .............................................................................................. 4-1 4-1

II. Coated optics ........................................................................................................ 4-13 4-31III. General construction features............................................................................... 4-17 4-35

CHAPTER 5. ELEMENTARY PRINCIPLES OF TELESCOPESI. Introduction ........................................................................................................... 5-1 5-1II. Astronomical telescopes ....................................................................................... 5-5 5-2III. Terrestrial telescopes............................................................................................ 5-8 5-4IV. Functions of collective lenses and eyepieces ....................................................... 5-12 5-8V. Adjustments .......................................................................................................... 5-14 5-10VI. Functions of telescopic sights ............................................................................... 5-17 5-12VII. Optical factors in telescope design ....................................................................... 5-22 5-13

CHAPTER 6. PRINCIPLES OF LASERS.................................................................................... 6-1 6-1

CHAPTER 7. INFRARED PRINCIPLES ..................................................................................... 7-1 7-1

*This manual supersedes TM 9258, May 1966.

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Paragraph Page

CHAPTER 8. TYPICAL FIRE CONTROL INSTRUMENTSSection I. Telescopes............................................................................................................ 8-1 8-1

II. Articulated telescopes........................................................................................... 8-5 8-3III. Elbow telescopes .................................................................................................. 8-8 8-5IV. Panoramic telescopes........................................................................................... 8-10 8-8V. Periscopes ............................................................................................................ 8-13 8-10VI. Binoculars ............................................................................................................. 8-18 8-16VII. Range finders........................................................................................................ 8-21 8-19VIII. Specialized pilitary optical instruments.................................................................. 8-24 8-26

CHAPTER 9. TESTING OPTICAL PROPERTIES OF INSTRUMENTS..................................... 9-1 9-1

CHAPTER 10. MEASUREMENT SYSTEMS EMPLOYED IN OPTICS........................................ 10-1 10-1

APPENDIX A. REFERENCES...................................................................................................... A-1

B. GLOSSARY .......................................................................................................... B-1

INDEX................................................................................................................................................. I-1

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TM 9-258LIST OF ILLUSTRATIONS

Figure Title Page1-1 Military applications of Optics ............................................................................................................................................... 1-22-1 Wave carrying energy along rope ......................................................................................................................................... 2-22-2 Wave train passing along rope ............................................................................................................................................. 2-32-3 Electromagnetic spectrum .................................................................................................................................................... 2-42-4 Light sources send waves in all directions ............................................................................................................................ 2-52-5 Light travels in direction of radii of waves.............................................................................................................................. 2-52-6 Waves and radii from nearby light source............................................................................................................................. 2-62-7 Waves and radii from distant light source ............................................................................................................................. 2-62-8 Speed of light ....................................................................................................................................................................... 2-72-9 Wavelength .......................................................................................................................................................................... 2-82-10 Frequency ............................................................................................................................................................................ 2-82-11 Symbols and types of illustrations used in this manual ......................................................................................................... 2-92-12 Beam of light reflected back ................................................................................................................................................. 2-102-13 Terms used with reference to reflected light ......................................................................................................................... 2-102-14 Reflection from a plane mirror .............................................................................................................................................. 2-112-15 Regular reflection ................................................................................................................................................................. 2-122-16 Irregular of diffuse reflection ................................................................................................................................................. 2-122-17 Reflection at optical interfaces .............................................................................................................................................. 2-122-18 Reflection from convex mirror, infinitely distant source ......................................................................................................... 2-132-19 Reflection from concave mirror ............................................................................................................................................. 2-142-20 Projection by spherical mirror ............................................................................................................................................... 2-142-21 Projection by parabolic mirror ............................................................................................................................................... 2-152-22 Path of beam of light through sheet of glass ......................................................................................................................... 2-152-23 Effects of refraction .............................................................................................................................................................. 2-162-24 Terms used with reference to reflected light ......................................................................................................................... 2-172-25 Index of refraction................................................................................................................................................................. 2-182-26 Atmospheric refraction bends sun rays................................................................................................................................. 2-192-27 Sun below horizon seen by reflected light............................................................................................................................. 2-192-28 Path of light ray through prism.............................................................................................................................................. 2-202-29 Deviation of rays by two prisms, base to base ...................................................................................................................... 2-212-30 Deviation of rays by convergent lens .................................................................................................................................... 2-212-31 Paths of rays of light through convergent lens ...................................................................................................................... 2-222-32 Law of refraction applied to lens ........................................................................................................................................... 2-222-33 Deviation of rays by two prisms, apex to apex ...................................................................................................................... 2-232-34 Deviation of rays by divergent lens ....................................................................................................................................... 2-232-35 Angles of light rays from an underwater source .................................................................................................................... 2-242-36 Total internal reflection in 90 degree prism ........................................................................................................................... 2-252-37 Virtual image reflected by plane mirror ................................................................................................................................. 2-262-38 Real image formed on ground glass of camera .................................................................................................................... 2-262-39 Letter F as reflected or refracted by optical elements ........................................................................................................... 2-272-40 Reflection of point of light in mirror ....................................................................................................................................... 2-282-41 Reflection of letter F in mirror ............................................................................................................................................... 2-292-42 Image transmission by mirrors.............................................................................................................................................. 2-302-43 Image transmission by prism ................................................................................................................................................ 2-302-44 Focal lengths of convergent lens .......................................................................................................................................... 2-312-45 Optical center of the lens ...................................................................................................................................................... 2-312-46 Focal length of divergent lens ............................................................................................................................................... 2-322-47 Image formed by convergent lens......................................................................................................................................... 2-332-48 Magnification by reading glass-object within focal length ...................................................................................................... 2-352-49 Effect of parallel rays on divergent lens ................................................................................................................................ 2-362-50 Reduction by divergent (negative) lens................................................................................................................................. 2-362-51 Dispersion ............................................................................................................................................................................ 2-382-52 Field of view limited by optical possibilities of eyepiece ........................................................................................................ 2-392-53 Comparison of light entering eye with 2-power telescope ..................................................................................................... 2-402-54 Exit pupil............................................................................................................................................................................... 2-412-55 Eye distance......................................................................................................................................................................... 2-412-56 Spherical aberration of convergent lens................................................................................................................................ 2-432-57 Spherical aberration of divergent lens................................................................................................................................... 2-432-58 Spherical aberration reduced................................................................................................................................................ 2-442-59 Effect of compound lens on spherical aberration .................................................................................................................. 2-442-60 Cause of chromatic aberration in lens................................................................................................................................... 2-452-61 Correction of chromatic aberration of lens ............................................................................................................................ 2-452-62 Correction of chromatic aberration of prism .......................................................................................................................... 2-46

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LIST OF ILLUSTRATIONS-Continued

Figure Title Page2-63 Proper refraction of light ....................................................................................................................................................... 2-472-64 Astigmatic refraction of light ................................................................................................................................................. 2-482-65 Formation of coma, greatly magnified................................................................................................................................... 2-492-66 Appearance of coma, greatly magnified................................................................................................................................ 2-492-67 Barrel-shaped (A and B) and hourglass (C and D) distortion ................................................................................................ 2-502-68 Hourglass distortion in magnification by reading glass.......................................................................................................... 2-512-69 Curvature of image ............................................................................................................................................................... 2-522-70 Diffraction pattern, greatly magnified .................................................................................................................................... 2-542-71 Newton’s rings...................................................................................................................................................................... 2-542-72 Parallax ................................................................................................................................................................................ 2-563-1 Comparison of eye and camera............................................................................................................................................ 3-13-2 Cross section of eyeball........................................................................................................................................................ 3-23-3 Action of iris compared with camera diaphragm ................................................................................................................... 3-43-4 Suspension and action of crystalline lens ............................................................................................................................. 3-53-5 Cones and rods of retina ...................................................................................................................................................... 3-63-6 Field of view of the eyes ....................................................................................................................................................... 3-83-7 Cube as seen by left eye, right eye, and both eyes............................................................................................................... 3-103-8 Judging distance................................................................................................................................................................... 3-123-9 Angular discernible difference............................................................................................................................................... 3-133-10 Nearsightedness (myopia).................................................................................................................................................... 3-143-11 Farsightedness (hypermetropia) ........................................................................................................................................... 3-153-12 Visual Limitations ................................................................................................................................................................ 3-164-1 Types of simple lenses ......................................................................................................................................................... 4-24-2 Comparative focal lengths of elements in compound lens .................................................................................................... 4-34-3 Types of objectives............................................................................................................................................................... 4-44-4 Typical light paths through field lens and eyelens of eyepiece.............................................................................................. 4-54-5 Kellner and ramsden eyepieces............................................................................................................................................ 4-64-6 Huygenian and symmetrical eyepieces................................................................................................................................. 4-84-7 Erfle and orthoscopic eyepieces ........................................................................................................................................... 4-104-8 French plossl, and variable magnification eyepieces ............................................................................................................ 4-114-9 Porro prism system .............................................................................................................................................................. 4-124-10 Abbe system......................................................................................................................................................................... 4-124-11 Amici or roof-angle prism...................................................................................................................................................... 4-134-12 Rhomboidal prism................................................................................................................................................................. 4-134-13 Rotating prisms .................................................................................................................................................................... 4-154-14 Pentaprism (diagrams show effect obtained with mirrors)..................................................................................................... 4-174-15 Triple mirror prism ................................................................................................................................................................ 4-184-16 Double right-angle Abbe prism ............................................................................................................................................. 4-194-17 Rotation of wedge changes direction of path of light............................................................................................................. 4-194-18 Extent of displacement of light may be changed by movement of wedge ............................................................................. 4-204-19 Principles of rotating measuring wedges .............................................................................................................................. 4-214-20 Lens erection systems.......................................................................................................................................................... 4-224-21 Erecting system using two double right-angle abbe prisms................................................................................................... 4-234-22 Optical system of panoramic telescope ................................................................................................................................ 4-244-23 Image rotation in dove prism (end view) ............................................................................................................................... 4-254-24 Representative types of reticles............................................................................................................................................ 4-264-25 Representative types of reticle patterns................................................................................................................................ 4-274-26 Diaphragm (stop)location...................................................................................................................................................... 4-284-27 Types of ray filter mountings................................................................................................................................................. 4-294-28 Principles of polarization of light ........................................................................................................................................... 4-304-29 Instrument light for panoramic telescope .............................................................................................................................. 4-304-30 Instrument light for BC periscope.......................................................................................................................................... 4-314-31 Light ray striking uncoated surface ....................................................................................................................................... 4-324-32 Light ray striking coated surface (coating one-half wavelength in thickness)......................................................................... 4-334-33 Light ray striking coated surface (coating one-quarter wavelength in thickness) ................................................................... 4-334-34 Low-reflectance coating........................................................................................................................................................ 4-344-35 Lens cell, separators, lenses and retaining ring .................................................................................................................... 4-354-36 Eccentric objective mounting ................................................................................................................................................ 4-364-37 Sunshade and objective cap................................................................................................................................................. 4-364-38 Diopter scale ........................................................................................................................................................................ 4-375-1 Simple magnifier-object at principal focus............................................................................................................................. 5-15-2 Simple magnifier-object with focal length o ........................................................................................................................... 5-1

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TM 9-258LIST OF ILLUSTRATIONS-Continued

Figure Title Page5-3 Simple compound microscope.............................................................................................................................................. 5-25-4 Objective lens....................................................................................................................................................................... 5-35-5 Keplerian system.................................................................................................................................................................. 5-35-6 Refracting astronomical telescope........................................................................................................................................ 5-35-7 Concave mirror ..................................................................................................................................................................... 5-35-8 Reflecting astronomical telescope ........................................................................................................................................ 5-45-9 Terrestrial telescope-simple form.......................................................................................................................................... 5-45-10 Symmetrical erectors............................................................................................................................................................ 5-55-11 Simple erector ...................................................................................................................................................................... 5-55-12 Asymmetrical erectors .......................................................................................................................................................... 5-55-13 Galilean system.................................................................................................................................................................... 5-75-14 Relation of elements in galilean telescope (zero Diopter setting) .......................................................................................... 5-75-15 Objective diameter limits field in galilean system .................................................................................................................. 5-85-16 Function of collective lens..................................................................................................................................................... 5-95-17 Function of eyepiece ............................................................................................................................................................ 5-95-18 Function of field lens in eyepiece.......................................................................................................................................... 5-105-19 Zero Diopter setting .............................................................................................................................................................. 5-105-20 Minus setting for shortsighted eye ........................................................................................................................................ 5-105-21 Plus setting for farsighted eye .............................................................................................................................................. 5-105-22 Dioptometer.......................................................................................................................................................................... 5-115-23 Reticle location ..................................................................................................................................................................... 5-125-24 Entrance and exit pupils ....................................................................................................................................................... 5-135-25 Exit pupil-virtual image of objective....................................................................................................................................... 5-145-26 Ramsden dynameter ............................................................................................................................................................ 5-145-27 Field limit .............................................................................................................................................................................. 5-155-28 Linear true field..................................................................................................................................................................... 5-155-29 Eye-relief-symmetrical erectors with real images at focal points of erectors and image 0 of objective between erectors ...... 5-165-30 Eye relief-objective image outside erectors........................................................................................................................... 5-165-31 Eye relief-collective lens in system ....................................................................................................................................... 5-165-32 Angular limit of resolution ..................................................................................................................................................... 5-176-1 Gas laser .............................................................................................................................................................................. 6-16-2 Solid-state red lasers............................................................................................................................................................ 6-27-1 Photoconductive detector ..................................................................................................................................................... 7-27-2 Bolometer circuit................................................................................................................................................................... 7-38-1 Telescope (rifle)--Assembled view, reticle pattern................................................................................................................. 8-18-2 Telescope (sniper’s sighting device)--assembled view.......................................................................................................... 8-28-3 Observation telescope--assembled view with tripod.............................................................................................................. 8-28-4 Observation telescope-optical elements and optical diagram ............................................................................................... 8-38-5 Articulated telescope-optical system diagram ....................................................................................................................... 8-38-6 Articulated telescope (tank)--assembled view....................................................................................................................... 8-48-7 Articulated telescope (self propelled vehicle) --assembled view............................................................................................ 8-48-8 Schematic diagram of elbow telescope................................................................................................................................. 8-58-9 Elbow telescope (howitzer) -assembled view, reticle patterns............................................................................................... 8-68-10 Elbow telescope (sight units)--assembled view, reticle pattern ............................................................................................. 8-78-11 Elbow telescope (towed howitzer)--assembled view ............................................................................................................. 8-78-12 Elbow telescope (self propelled howitzer)--assembled view.................................................................................................. 8-88-13 Panoramic telescope (towed howitzer)-assembled view, optical elements, and optical diagram........................................... 8-98-14 Panoramic telescope (self propelled howitzer) --assembled view ......................................................................................... 8-108-15 Periscope (tank observation)--assembled view, optical elements and diagram of principle of operation ............................... 8-118-16 Optical diagram and optical elements of periscope............................................................................................................... 8-128-17 Periscope (self propelled vehicle)--assembled view.............................................................................................................. 8-138-18 Periscope (tank)--assembled view........................................................................................................................................ 8-138-19 Periscope (tank-diagram of optical system ........................................................................................................................... 8-148-20 Periscope (infrared) -assembled view................................................................................................................................... 8-158-21 Binocular (general use)-assembled view, optical elements, and optical diagram .................................................................. 8-178-22 Battery commander’s periscope-assembled view, reticle pattern.......................................................................................... 8-198-23 Optical system schematic ..................................................................................................................................................... 8-208-24 Fundamental triangle of range finder .................................................................................................................................... 8-218-25 Range finder, tank M17A1 and M17C (typical coincidence type) .......................................................................................... 8-228-26 Range finder, reticle patterns................................................................................................................................................ 8-258-27 Collimator sight-optical system and optical diagram ............................................................................................................. 8-278-28 Reflex sight........................................................................................................................................................................... 8-288-29 Aiming circle ......................................................................................................................................................................... 8-29

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LIST OF I LLUSTRATIONS-Continued

Figure Title Page8-30 Laser range finder ................................................................................................................................................................ 8-308-31 Fire control subsystem ......................................................................................................................................................... 8-3210-1 Artillery mil-degree conversion chart..................................................................................................................................... 10-210-2 Metric unit-inch conversion table .......................................................................................................................................... 10-6B-1 Absorption-selective transmission ........................................................................................................................................ B-1B-2 Angle of azimuth................................................................................................................................................................... B-2B-3 Angle of elevation, angle of site, and quadrant angle............................................................................................................ B-3B-4 Apertures.............................................................................................................................................................................. B-4B-5 Astigmatism.......................................................................................................................................................................... B-5B-6 Axis of bore and boresights-boresighting.............................................................................................................................. B-6B-7 Burnishing ............................................................................................................................................................................ B-7B-8 Center of curvature............................................................................................................................................................... B-7B-9 Conjugate focal points (conjugate foci) ................................................................................................................................. B-9B-10 Converging lenses................................................................................................................................................................ B-9B-11 Diverging lenses................................................................................................................................................................... B-11B-12 Drift....................................................................................................................................................................................... B-12B-13 Lens erecting system............................................................................................................................................................ B-13B-14 Eyelens and field of eyepiece ............................................................................................................................................... B-14B-15 Focal length of lens and mirror ............................................................................................................................................. B-15B-16 Real image produced by converging lens ............................................................................................................................. B-17B-17 Virtual image produced by diverging lens ............................................................................................................................. B-17B-18 Index of refraction................................................................................................................................................................. B-18B-19 Optical system of erecting telescope .................................................................................................................................... B-19B-20 Linear field............................................................................................................................................................................ B-20B-21 Magnifying power.................................................................................................................................................................. B-21B-22 Nearsightedness (myopia).................................................................................................................................................... B-22B-23 Normal and oblique .............................................................................................................................................................. B-22B-24 Porro prism........................................................................................................................................................................... B-25B-25 Prism.................................................................................................................................................................................... B-25B-26 Protractor.............................................................................................................................................................................. B-26B-27 Reticle patterns superimposed on image of object ............................................................................................................... B-27B-28 Right angle prism.................................................................................................................................................................. B-28B-29 Waves and wave fronts ........................................................................................................................................................ B-30

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CHAPTER 1

INTRODUCTION

1-1. Purpose. This manual is published primarily forthe information and guidance of Army maintenance and

using personnel and others who must be familiar with thefunctioning of fire control instruments (fig 1-1).

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Figure 1-1. Military application of optics.

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1-2. Scopea. This manual covers the basic principles of

optical theory necessary to understand the operation offire control instruments. It contains sufficient descriptivematter and illustrations to provide a general knowledgeof the principles upon which the design and constructionof military optical instruments are based. A backgroundin physics and mathematics, though helpful, is not

essential, as the use of those subjects in this text is on asimple level.

b. The glossary contains definitions of specializedterms and unusual words used in this manual. A list ofcurrent references appears in this appendix.

c. The material presented herein is applicablewithout modification, to both nuclear and non-nuclear warfare.

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TM 9-258CHAPTER 2

PROPERTIES OF LIGHT

Section I. LIGHT2-1. Theories and Known Facts About Light.

a. General. The true nature of light and themanner in which it travels have fascinated scientificinvestigators for centuries. Many theories have beenadvanced to explain why light behaves as it does. Laterdiscoveries have proved some of these theoriesunsound. This manual cannot attempt to cover all theaspects of light nor is this knowledge essential to thestudy of the laws of optics as they apply to fire controlequipment. Therefore, only a short summary of thevarious theories of light is given.

(1) Ancient Greek theory. The earliest knownspeculations as to the nature of light were those of theancient Greeks. They believed that light was generatedby streams of particles which were ejected from thehuman eye. This theory could not persist because itcould not explain why one could not see as well at nightas by day.

(2) Newton. The first modern theory was thatof Sir Isaac Newton (1643-1727). This, the corpusculartheory, assumed light to be a flight of material particlesoriginating at the light source. Newton believed that lightrays moved at tremendous velocity in a state of nearvibration and could pass through space, air, andtransparent objects. This theory agreed with the propertyof light to move only in a straight line, in a medium ofconstant optical density, but failed to explain otherphases of light behavior. Newton stumbled on lightinterference in his discovery of Newton’s Rings (para 2-44)but did not realize their significance.

(3) Huygens. In Newton’s time, ChristianHuygens (1629-1695) attempted to show that the laws ofreflection and refraction of light could be explained by histheory of wave motion of light. While this theory seemedthe logical explanation for some phases of light behavior,it was not accepted for many years because a meanswas lacking by which to transmit the waves. Huygensproposed that a medium, which he called "ether, " beaccepted as existing to serve light rays as water does thefamiliar waves of water. He assumed this medium tooccupy all space, even that already occupied by matter.

(4) Young and Fresnel. Experiments madeabout 50 years later by Thomas Young (1773-1829),

Fresnel (1788-1827), and others supported the wavetheory and the rival corpuscular theory was virtuallyabandoned. These scientists offered their measurementof light waves as proof. They accepted the "ether" theoryand assumed light to be waves of energy transmitted byan elastic medium ether.

(5) Maxwell, Boltzmann, and Hertz. Light andelectricity were similar in radiation and speed was provedthe experiments of Maxwell (18311879), Boltzmann(1844-1906), and Hertz (18571894). From theseexperiments was developed the Electromagnetic Theory.These experiments produced alternating electric currentsof short wavelengths that were unquestionably ofelectromagnetic origin and had all the properties of lightwaves. The Maxwell theory contended that energy wasgiven off continuously by the radiating body.

(6) Planck. For a time it was thought that thepuzzle of light had been definitely solved but, in 1900,Max Planck refuted the Maxwell theory that the energyradiated by an ideal radiating body was given offcontinuously. He contended that the radiating bodycontained a large number of tiny oscillators, possibly dueto the electrical action of the atoms of the body. Theenergy radiated could be high frequency (para 2-3b) andwith high energy value, all possible frequencies beingrepresented. The higher the temperature of the radiatingbody the shorter is the wave length (para 2-3a) of mostenergetic radiation. In order to account for the manner inwhich the radiation from a warm black body is distributedamong the different wave lengths, Planck found anequation to fit the experimental curves, and then only onthe very novel assumption that energy is radiated in verysmall particles which, while invisible, were grains ofenergy just as much as grains of sand. He called theseunits quanta and his theory the Quantum Theory. Theelementary unit or quanta for any given wave length isequal to hn, where n is frequency of the emitted radiationand h is a constant known as Planck’s constant. Quantaset free were assumed to move from the source inwaves.

(7) Einstein. A few years later, Albert Einsteinbacked up in Planck and contended that

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the light quanta when emitted retained their identity asindividual packets of energy.

(8) Millikan and Compton. Still later, R.A.Millikan’s very accurate measurements proved that theenergy due to motion (called "kinetic energy) ofelemental units of light (called "photons") behaved asassumed by the Quantum Theory. Later proof was givenby A.H. Compton in 1921 in determining the motion of asingle electron and a photon before and after collision ofthese bodies. He found that both have kinetic energyand a momentum and that they behave like materialbodies. This was somewhat a return to the oldcorpuscular theory.

(9) Summary. The present standpoint ofphysicists on the nature of light is that it appears to bedualistic, both particle and wavelike. It is assumed thatlight and electricity have much in common. It is presentlyaccepted that the energy associated with a light beam istransmitted as small particle-like packets-photons,originally called quanta (para 2-1a (6) which may bedescribed in terms of associated waves, but, which arebest measured relying solely on their particle-like nature.The simple wave analogy is only a very roughapproximation. However, the phenomena of lightpropagation may be best explained in terms of wavetheory. The wave theory will be used in the remainder ofthis manual.

b. Known Facts. All forms of light obey the samegeneral laws. Light travels in waves in straight lines andat a definite and constant speed, provided it travels in a

medium or substance of constant optical density. Uponstriking a different medium, light either will bounce back(be reflected) or it will enter the medium. Upon enteringa transparent medium, its speed will be slowed down ifthe medium is more dense or increased if the medium isless dense. Some substances of medium density haveabnormal optical properties and for this reason "opticallydense" would be a better term. If it strikes the medium atan angle, its course will be bent (refracted ) uponentering the medium. Upon striking all materialmediums, more or less of the light is absorbed.

(1) Transmission of light energy. Visible lightis one of many forms of radiant energy which istransmitted in waves. The means by which the wavescarry the energy can be illustrated in a simple manner.Secure one end of a rope to some object. Hold the otherend of the rope, stretch it fairly taut, and shake it. Awave motion (wave pulse) will pass along the rope fromthe end that is held to the end that is secured (fig 2-1). Ifthe end of the rope is continually shaken, a series ofwaves (wave train) will pass along the rope (fig. 2-2). Itwill be noted that the different parts of the rope (themedium) vibrate successively, each bending back andforth about its own position. The disturbance travels butthe medium does not. Only the energy is carried alongthe rope. Now imagine that the light source is a vibratingball from which a countless number of threads extend inall directions. As the ball vibrates, successive waves aretransmitted out along the threads in all directions. In asimilar manner, light radiates from its source.

Figure 2-1. Wave carrying energy along rope.

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Figure 2-2. Wave train passing along rope.

(2) Electromagnetic spectrum. Heat and radiowaves, light waves, ultraviolet and infrared rays, X-rays,and cosmic rays are forms of radiant energy of differentwavelengths and frequencies. Together, they form whatis known as the electromagnetic spectrum (fig 2-3). Thevisible light portion of the electromagnetic spectrumconsists of wavelengths from 0.00038 to 0.00066millimeter. The different wavelengths represent different

colors of light. Practically all light is made up of manycolors, each color having its distinctive wavelength andfrequency. In as much as light of each color reacts in aslightly different manner when passed through differentmediums, provision must be made in optical elements tocontrol the action of light of various colors.

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Figure 2-3. Electromagnetic spectrum.

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(3) Light rays. In considering visible light,assume that it starts from a luminous point and travelsoutwardly in all directions in waves through a medium ofconstant density to form a sphere of which the luminouspoint is the center (fig 2-4). The direction in which thelight is traveling is along the radii of the sphere of light(fig 2-5) at right angles to the fronts of the waves. Lighttraveling along these radii is termed light rays.

Figure 2-4. Light sources send waves in all directions.

Figure 2-5. Light travels in direction of radii of waves.

(4) Light rays from distant source. The wavefront radiating from a light source is curved when it isnear the source (fig 2-6) and the radii of the wavesdiverge or spread. The wave front becomes less andless curved as the waves move outwardly, eventuallybecoming almost straight (fig 2-7), and the radii of wavesfrom a distant source are virtually parallel.

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Figure 2-6. Waves and radii from nearby light source.

Figure 2-7. Waves and radii from distant light source.

(5) Principle of reversibility of light paths. If asingle narrow beam of parallel light is reflected orrefracted in any combination through a number of opticalelements, it will retrace its path through the elements ifthe light were to enter the optical system from the otherend. This is true no matter how many reflections andrefractions the light has undergone.

(6) Color. Most substances reflect lightselectively. Certain wavelengths (colors) are reflected,while others are absorbed. The color of an object is theone which is reflected. A colored transparent medium(filter) transmits light selectively, absorbing all rays nottransmitted.

(7) Biological effects. Infrared light is not veryeffective on living tissues except to produce heat. Visiblelight is beneficial in its effects. Shorter waves mayproduce serious burns, but possess marked bactericidalproperties and are used for sterilizing milk and othermaterials. In general, the shorter the waves the moreviolent are they in their effects on living tissues.2-2. Speed of Light

a. The speed of light (fig 2-8) is an important factorin the study of the physical nature of light., The lengths ofall waves in the electromagnetic spectrum (fig 2-3) arelogically connected to corresponding frequencies and tothe speed of light (para 2-3). The difference in the speedof light through air, glass, and other substances accountsfor the bending of light rays or refraction.

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Figure 2-8. Speed of light

b. Light travels so fast that it was a long time beforeits speed could be measured. Galileo Galilei (1564-1642) tried to measure light velocity by means of blinkinglanterns on widely distant hilltops but, as the speed wastoo great to be measured by this method, the velocity oflight was presumed to be infinite.

c. Thirty-four years after Galileo’s death, OlausRoemer (1644-1710) determined the approximate speedof light by astronomical means based on observationswhich showed the amount of time required for the light ofthe planet Jupiter to cross the orbit of the earth.

d. It was nearly 200 years more before scientistswere able to measure a speed so great as that of light bylaboratory methods. In 1849, Fizeau (1819-1896)measured the velocity of light, using intermittent flashesof light sent out by a source equipped with a swiftlymoving toothed wheel and reflected back to theobserver. Other methods, employing the reflectionsfrom rotating mirrors and prisms, have since been used

to establish the speed of light to an exactness beyondthat required for practical usage.

e. The speed of light is accepted as 186, 330 milesper second. This is the figure used in computationsinvolving the speed of light. The speed of light obtainedby the important methods of the past 70 years rangesfrom 186, 410 to 186, 726 miles per second.2-3. Wavelength and Frequency

a. Wavelength. The wavelength is the distancefrom the crest or top of one wave to the crest of the next,or from any point on one wave to the correspondingpoint, in the same phase on the next wave (fig 2-9). Thevelocity of all forms of electromagnetic waves in vacuumis the same, that is, the speed of light is approximately186, 000 miles per second. The wavelengths, however,vary greatly(fig 2-3).

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Figure 2-9. Wavelength

b. Frequency. The frequency is the number ofwaves occurring per second. It is determined by dividingthe speed of light by the wavelength. It is the number ofwaves passing a given point in 1 second (fig 2-10).

VelocityFrequency =

Wavelength

Figure 2-10. Frequency

c. Special Units of Measurement Employedin Measuring Wavelengths. Wavelengths ofelectromagnetic waves range from many miles long tothose measured in trillionths of an inch. Themeasurements cover such a wide range and the shorterwavelengths are so minute that special units ofmeasurements have been provided to avoid longdecimal fractions of millimeters or inches.

(1) The first of the special units ofmeasurement is the micron, which is abbreviated to µ,the Greek letter "mu". It represents one one-millionth ofa meter or one one-thousandth of a millimeter. The nextis variously called the milli-micron and micro-millimeter.It is abbreviated to mµ and represents one one-thousandth of a micron. Finally, the micro-micron isabbreviated to go. It represents one one-millionth of amicron.

(2) Another important measurement is theAngstrom unit (AU) which is one-tenth of a milli-micronor one ten-millionth of a millimeter. Even Angstrom unitsare inconveniently long in measuring the shortestelectromagnetic waves so the X-ray unit (XU) is used forthis purpose. It is one one-thousandth of an Angstromunit.2-4. Light Rays and Other Symbols Used in theDiagrams.

a. Every point on a luminous body or an illuminatedobject sends out a constant succession of wave fronts inall directions. The action of light in passing through alens, for example, might be as shown in A, figure 2-11.But for simplicity, light will be indicated in this manual byone, two, or more representative "light rays." These "lightrays" are shown as lines with arrowheads indicating thedirection of travel and are shown as in B, figure 2-11.Wherever possible light will be indicated as coming fromthe left side of an illustration.

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Figure 2-11. Symbols and types of illustrations used in this manual.

b. Single rays of light do not exist. The term "lightray" is used throughout this manual for the sake of clarityand convenience to indicate the direction of travel oflight.

c. In studying the diagrams, it must beremembered that printed illustrations have only heightand width (b, fig 2-11) while everything, including light,has three dimensions: height, width, and depth (orlength). The reader’s imagination must be employed togive the illustration depth. Thus, for example, where twolight rays are used to indicate the course of light

from a luminous or illuminated point, these lightraysindicate a solid cone of light from that point (A, fig 2-11).In certain of the illustrations, an attempt has been madeto simulate height, width, and depth by preparing thediagrams to appear as though viewed from an angle (A,fig 2-11).

d. The letter F is used as the object in a great manyof the diagrams because it shows quickly whether theimage of this letter or object is upright, inverted, or reverted.

Section II. REFLECTION

2-5. Reflection From a Plane Mirror.a. A plane mirror is a flat polished surface used to

reflect light. If a beam of light is permitted to enter adarkened room and a plane mirror is held so that thebeam will strike the mirror, the beam will be reflected. Byshifting the mirror, the beam can be reflected to almostany part of the room.

b. If the mirror is shifted so that it is exactly at rightangles (normal) to the beam, the reflected beam will be

directed back along the path of the incoming beam (A,fig 2-12). If the mirror is shifted to an angle from thisposition, the reflected beam will be shifted at an anglefrom the incoming beam that is twice as great as theangle by which the mirror is shifted (B, fig 2-12). Forexample, if the mirror is held at angle of 45° to theincoming beam, the reflected beam will be projected atan angle of 90° to the incoming beam.

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Figure 2-12. Beam of light reflecting back.

c. These simple experiments illustrate one of thedependable forms of action of light. Light can bereflected precisely to the point where it is requiredbecause any kind of light, on being reflected by asmooth, polished surface, acts in the same manner.This action of light is put to use in many types of fire-control instruments.

d. The ray of light which strikes the surface iscalled the incident ray (fig 2-13). The ray which is

reflected is termed the reflected ray. An imaginary line atright angles to the surface is called the normal orperpendicular. The angle between the incident ray andthe normal is the angle of incidence, while the anglebetween the reflected ray and the normal is the angle ofreflection.

Figure 2-13. Terms used with reference to reflected light.

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e. The best reflectors are very smooth metalsurfaces, some of which may reflect as much as 98percent of perpendicular light. The glass in a silveredmirror serves only as a very smooth supporting surfaceand protective window for the silver coating on the back.In front surface aluminized mirrors, the glass serves onlyas a smooth supporting surface for the aluminumreflecting surface. The resulting aluminum oxide (fromthe oxidation of the aluminum when exposed to theoxygen in the air) is both colorless and transparent andso does not interfere to any appreciable extent with thepassage of light. This type of coating is used on thereflectors in the huge reflecting astronomical telescopes.

2-6. Law of Reflectiona. The law of reflection is as follows: The angle of

reflection is equal to the angle of incidence and lies onthe opposite side of the normal; the incident ray,reflected ray, and normal all lie in the same plane.

b. Diagrams A, B, C, and D in figure 2-14 showlight rays incident upon plane mirrors at successivelysmaller angles. By applying the law of reflection, it canbe seen that in all such cases of reflection the angle ofreflection can be plotted as long as the angle ofincidence is known or vice versa.

Figure 2-14. Reflection from a plane mirror.

2-7. Types of Reflection.a. There are two main types of reflection regular

and irregular.

b. Regular reflection occurs when light strikes asmooth surface and is reflected in a concentrated manner (fig 2-15).

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Figure 2-15. Regular reflection.

c. If a beam of light strikes a rough surface, suchas a sheet of unglazed paper or ground glass, the light isnot reflected regularly but is scattered in all directions (fig2-16). This is called irregular reflection or diffusion.

Figure 2-16. Irregular or diffuse reflection.

d. The direction in which any single ray of light willbe reflected can be plotted by erecting a perpendicular ornormal at the point of impact and applying the law ofreflection (fig 2-16). Any rough surface can be regardedas an almost infinite number of elementary planesurfaces at each of which the law of reflection holds true.

e. Light is reflected by diffusion or by regularreflection and diffusion from nearly every object which isseen. Because the light falling on such an object is, inpart, scattered in all directions, the object is made visible,generally, when viewed from any direction. Skin, fur, andevery dull surface reflect light in essentially a diffusedmanner. The glossy finish of a new automobile or thebright finish of a polished casting reflect light essentiallyby regular reflection and partially by diffusion.

f. Surfaces of transparent mediums such as opticalinterfaces also will reflect. The amount of incident lightreflected depends upon the angle of incidence. As theangle of incidence increases, so does the amount ofreflected light. When light passes through a piece ofglass, reflection occurs at both surfaces as in figure 2-17. Approximately 4 percent of the incident light is lost orreflected at the first surface and another 4 percent of theemergent light is lost by reflection at the second surface.Thus, both reflection and refraction (the bending of light)occur at any optical interface. The importance to opticalsystems is discussed in detail in paragraphs 4-1 through4-16).

Figure 2-17. Reflection at optical interfaces

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2-8. Reflection from Convex Mirror.a. The law of reflection holds true for all surfaces

whether convex, concave, or plane. The action ofvarious curved surfaces in directing the course of thereflected light will vary according to the amount ofcurvature of the reflecting surface and the overall effectwill be dependent on the distance of the light source fromthe reflecting surface.

b. Assume that light from a practically infinitelydistant source, such as the sun (practically parallel rays),strikes a convex mirror (fig 2-18). It will be found that the

rays will be deflected in a divergent manner. The reasonfor this can be, determined by plotting the angles ofreflection of individual rays in relation to their angles ofincidence and the normals for each ray. In this case, thenormal for each ray is an imaginary line drawn from thecenter of curvature of the mirror to the point of incidenceof the ray. The angle of reflection will be equal to theangle of incidence for each ray.

Figure 2-18. Reflection from convex mirror, infinitely distant source.

c. If the light source is at a finite distance, or close to the mirror, the rays will be reflected in a divergent manner also.Such rays will be reflected at different angles than would be the case if all of them struck the mirror as parallel rays.2-9. Reflection from Concave Mirror.

a. The law of reflection is again applied to locate the path of the reflected rays when plotting the reflection from aconcave mirror. In this instance, the center of curvature of the mirror is in front of the mirror (fig 2-19). Imaginary lines arerun from this center to the points of incidence of the incident rays, to indicate the normals of individual rays. When this hasbeen done, the reflected rays can be plotted so that each forms an angle of reflection which will be equal to the angle ofincidence of the corresponding ray.

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Figure 2-19. Reflection from concave mirror.

b. It will be noted that the rays originating from aninfinite (distance) source, such as the sun, areconverging after reflection and that they intersect veryclose to a point halfway between the center of curvatureand the surface of the mirror. This point is known as theprincipal focal point of the mirror. This point of principalfocus is always one-half of the distance from the centerof curvature to the surface of a concave mirror, providedthe mirror is a true portion of a sphere. Due to "sphericalaberration, " the principal focus is a true point only for asmall central bundle of rays. After passing through thefocal point, the light will diverge.

c. If a very small luminous source is located at theprincipal point of focus, the rays will be nearly parallelafter reflection. This is true only if the curvature is veryslight. Actually, the rays will have a slight convergence,especially those which are reflected from near the edgesof the mirror (fig 2-20). For this reason, if it is desired tohave practically parallel rays after reflection, a parabolicmirror is used.

Figure 2-20. Projection by spherical mirror.

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d. With a parabolic mirror, light rays emanatingfrom an infinitely small source located at the focal pointwould be parallel after reflection, since this is theprincipal focal point of the parabolic mirror (fig 2-21). Inpractice, however, the source of light, which may be afilament or an arc, is located in the principal point offocus and the rays diverge as there cannot be a truepoint source. All rays falling upon the parabolic mirror,except those which are diffused or scattered, arereflected nearly parallel to each other. This allows a longpowerful beam of light to be formed which diverges veryslightly.

Figure 2-21. Projection by parabolic mirror.

Section III. REFRACTION

2-10. Refraction Through a Glass Plate.a. What happens when light is refracted can be

understood by imagining a beam of light, indicated by abroad "light ray" or "light beam" (fig 2-22), passing inslow motion" through a sheet of glass. Both plane

surfaces of this plate are parallel and air contacts bothsurfaces. The glass and the air are transparent but theglass is optically denser than the air. Light travelsapproximately one-third slower in glass than in air.

Figure 2-22. Path of beam of light through sheet of glass.

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b. If the beam strikes the glass directly at rightangles (or the normal), the light is not bent or refracted(A, fig 2-22). Its speed is slowed down uponencountering the’ optically denser medium, but as theentire front strikes the glass at the same instant its pathis not deviated but continues through the medium in astraight line. Upon reaching the other surface of theglass, the beam enters the optically lighter medium, air,and resumes its faster speed in air.

c. If this beam of light strikes the glass at an angle,one of the edges of the wave front arrives at the surfacean instant before the other edge does and consequentlyits path through glass is longer, at this point, whileentering glass. The edge of the front arriving at the glassfirst is slowed down upon entering the optically densermedium (B, fig 2-22). When the other edge enters theglass, it is slowed down in the same manner but aninstant later, having traveled with greater speed in air,the effect is to swing the entire beam toward the normal.This bending of light is termed refraction.

The light follows its new course in a direct line when theoptical density of the medium is constant.

d. Upon leaving the other surface of the glassplate, the light is again refracted or bent (B, fig 2-22).The edge which entered the glass first traverses theglass first. Upon leaving the optically denser mediumand entering the optically lighter medium (air), its path isdeviated again. The effect is to swing the entire beamaway from the normal.

e. Refraction may be visually demonstrated byplacing the straight edge of a sheet of paper at an angleunder the edge of a glass plate held vertically (B, fig 2-23). The straight edge of the sheet of paper will appearto have a jog in it directly under the edge of the glassplate. That portion of the paper on the other side of theglass will appear to be displaced due to refraction. Asthe sheet of paper is moved to change the angle of itsstraight edge, the amount of refraction will be increasedor decreased. If the straight edge of the paper is viewedat right angles to the surface of the glass, there will be norefraction.

Figure 2-23. Effects of refraction.

2-11. Law of Refraction.a. The law of refraction can be stated as follows: In

passing from a medium of lesser optical density to one ofgreater optical density, the path of light is deviated

toward the normal. In passing from a medium of greateroptical density to a medium of lesser optical density, thepath of light is deviated away from the normal (fig 2-24).

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Figure 2-24. Terms used with reference to refracted light.

b. The greater the angle at which the light strikesthe medium and the greater the difference in opticaldensity between the two mediums, the greater thebending. If the faces of the medium are parallel, thebending at the two faces is always the same so that thebeam which leaves the optically denser medium isparallel to the incident beam (A, fig 2-23).

c. The ray which strikes the surface is called theincident ray (fig 2-24). The ray entering the secondmedium is the refracted ray. The ray leaving the secondmedium is the emergent ray. An imaginary line at rightangles to the surface of the medium at the point wherethe ray strikes or leaves the surface is termed the normalor perpendicular. The angle between the incident rayand the normal is the angle of incidence. The anglebetween the refracted ray and an extension of theincident ray is termed the angle of deviation. It is theangle through which the refracted ray is bent from its

original path by the optical density of the refractingmedium. The angle formed by the refracted ray with thenormal is called the angle of refraction. The incident ray,refracted ray, emergent ray, and the normal all lie in thesame plane (fig 2-24).

d. The angle of refraction depends on the relativeoptical density of the two mediums as well as on theangle of incidence.

e. An equation known as Snell’s law may be usedto determine the angle of refraction (r) if the angle ofincidence (i) and the index of refraction (para 2-12c) ofeach medium are known (fig 2-25).

sin iThis equation is = n where n is the sin r

sin rquotient of the indices of refraction of the two mediums.If one of the mediums is air, n is practically the index ofrefraction of the second medium.

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Figure 2-25. Index of Refraction.

2-12. Index of Refraction.a. Light travels through substances of different

optical densities with greatly varying velocities. Forexample, the speed of light in air is approximately 186,000 miles per second; in ordinary glass, it isapproximately 120, 000 miles per second.

b. The ratio between the speed of light in vacuumand the speed of light in a medium is known as the indexof refraction. The index of refraction is usually indicatedby letter n and is determined by dividing the speed oflight in a vacuum by the speed of light in the particularmedium.

Velocity in VacuumIndex of Refraction =

Velocity in MediumThe index of refraction n, of a substance, can be

determined from the equation of Snell’s law (para 2-11e)by actually measuring the angles i and r in a simpleexperiment, inasmuch as air can be used instead ofvacuum for all practical purposes.

c. The following listing contains the indices ofrefraction of a number of substances.

NOTEFor most computations, the index of airis considered to be unity (1.000). Theerror introduced equals approximately0.03 percent.

Vacuum ....................................................1.000000Air .............................................................1.000292Water ..............................................................1.333Boro-silicate crown glass................................1.517Thermosetting cement....................................1.529Thermoplastic cement

(Canada balsam) .....................................1.530Gelatin ............................................................1.530Light flint glass................................................1.588Medium flint glass...........................................1.617Dense flint glass .............................................1.649Densest flint glass ..........................................1.963

2-13. Atmospheric Refractiona. At a surface separating two media of different

indices of refraction, the direction of the path of lightchanges abruptly when passing through the surface. Ifthe index of refraction of a single medium changesgradually as the light proceeds from point to point, thepath of the light will also change gradually and will becurved rather than in a straight line.

b. Although air at its densest has a refractive indexon only 1.000292, this is sufficient to bend light rays fromthe sun toward the earth when these rays strike theatmosphere at an angle (fig 2-26). The earth’satmosphere is a medium which becomes denser towardthe surface of the earth. The result is that of lighttraveling through the atmosphere toward the earth at anangle does not travel in a straight line but is refractedand follows a curved path. From points near the horizon,the bending of light is so great that the setting sun inseen after it is completely below the horizon (fig 2-27).

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Figure 2-26. Atmospheric refraction bends sun rays.

Figure 2-27. Sun below horizon seen by refracted light.

c. The bending of the paths of rays of light in bothvertical and a horizontal plane is due to variation of therefractive index of air due to moisture and temperaturevariations. This variation is generally so small that it canbe neglected in many types of observation made withfire-control instruments. In very careful surveys oflarge areas, this effect is eliminated by repeating the

observations on different days and under differentconditions. In the aiming of guns over ]wide expanses ofwater as must be done by coast artillery, a specialinstrument (the depression position finder) takesatmospheric refraction as well as other conditions intoconsideration.

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d. Over large areas of heated sand or over water,conditions are such as to produce strata or layers of airdiffering greatly in temperature and refractive index.Under such conditions, erect or inverted and sometimesmuch distorted images are formed which can be seenfrom a great distance. These images are known asmirages.

e. On a hot day, the columns of heated air risingfrom the earth are optically different from the surroundingair and rays of light are irregularly refracted. The air isturbulent and conditions under which observations aremade are changing all the time. Consequently, an objectviewed through such layers of air appears to be in motionabout a mean position. In such cases the air is said tobe "boiling" or the image is "dancing" due to "heatwaves." This condition is particularly detrimental when ahigh-power telescope is employed. Under suchconditions it is usually impossible to use an instrument ofmore than 20 power.2-14. Refraction Through Triangular Glass Prism.

a. Unlike a plate of glass, a prism has its faces cutat an angle. Refraction through a triangular glass prismdiffers from refraction through a sheet

of glass with parallel surfaces because the light whichpenetrates the base or thicker part of the prism musttravel longer and at less speed in an optically densermedium than the light passing through the apex orthinner part of the prism. Rays of light emerging from aplate of glass with parallel surfaces always travel in aparallel direction with the incident rays; rays emergingfrom a prism always travel at different angles from theincident rays.

b. The laws of refraction apply to prisms just asthey do to plates of glass with parallel surfaces. Theselaws may be applied to plot the paths of light through anyprism.

c. When a ray of light strikes the surface of aprism, the refracted ray (fig 2-28) is bent toward thenormal according to the law of refraction. The refractedray is bent away from the normal on leaving the prism.Thus, the path of the ray is deviated at the first surface ofthe prism and then further deviated at the secondsurface. In both cases, the ray is bent toward thethickest part of the prism.

Figure 2-28. Path of light ray through prism.

2-15. Refraction Through Lenses.a. Convergent Lens. If two prisms are

arranged base to base (fig 2-29), rays of light striking the

front surfaces will pass through the prisms. The raysemerging from one prism will cross the emergent rays ofthe other prism.

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Figure 2-29. Deviation of rays by two prisms, base tobase.

(1) If the two prisms are cut in semicircular form,their surfaces made spherical, and the two basescemented together, the result will be an optical elementknown as a convergent lens. (All convergent lenses arethicker at the center than at the edges. ) When parallelrays of light strike the front surface of a convergent lens(fig 2-30), all rays pass through the lens and converge toa single point where they cross (color and otheraberrations are disregarded at this time). Such a lensmay be thought of as consisting of an infinite number ofprisms arranged so that each directs light rays to thesame single point. The lens bends the rays as a prismdoes but, unlike a prism, it brings them to a point.

Figure 2-30. Deviation of rays by convergent lens

(2) Myriads of rays may be considered to come fromevery point of light on an object. Consider the refractionof three such rays from a point of light passing through aconvergent lens (fig 2-31) to intersect at a point on theother side of the lens. On the basis that a lens bends therays as a prism does, rays passing through the upperand lower portions of the lens would be bent toward thethickest part of the lens upon striking the first surfaceand bent again toward the thickest part in emerging. Asthe result, they would converge on the other side. Astraight line drawn through the center of the twospherical surfaces is termed the axis of the lens. Acentral or axial ray would not be deviated because itwould strike the surfaces of the lens at the normal. Sucha ray would join the outer rays at their convergent point.

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Figure 2-31. Paths of rays of light through convergent lens.

(3) The laws of refraction may be applied toplot the path of any ray through any lens. A ray enteringa lens will bend toward the normal of the lens at thatpoint (fig 2-32). The normal of an incident ray at anypoint on a lens is an imaginary line at right angles to thesurface of the lens at the

point where the ray enters. As the refracted ray leavesthe lens as the emergent ray, it is bent away from itsnormal. The normal of any emergent ray is an imaginaryline at right angles to the surface of the lens at the pointwhere the ray emerges from the lens.

Figure 2-32. Law of refraction applied to lens.

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b. Divergent Lens. A lens of a different type can beapproximated by placing two prisms apex to apex (fig 2-33). If rays of light strike the front surfaces of the prisms,the rays will pass through and, in accordance with thelaw of refraction, those passing through the upper prismwill travel upward while those passing through the lowerprism will travel downward. Now assume that the frontand rear surfaces of this pair of prisms have beenrendered spherical and that the two prisms have beenconverted into a lens (fig 2-34). All rays of light, exceptthe axial ray, passing through this lens would spread outor diverge but instead of splitting in two at the axis woulddiverge evenly in a spherical manner. This type of lensis known as a divergent lens. (All divergent lenses arethicker at the edges than at the center.

Figure 2-33. Deviation of rays by two prisms, apex toapex.

2-16. Total Internal Reflection and Critical Angle.

a. When light passing from air into a more opticallydense medium, such as glass, strikes the boundarysurface, it is refracted. This occurs regardless of theincident angle except when light is on the normal (norefraction occurs). However, when light attempting toleave a more optically dense medium for a less opticallydense medium strikes the boundary surface, it may bereflected rather than refracted even though bothmediums are perfectly transparent. This is known astotal internal reflection and occurs when the incidentangle exceeds a certain critical angle. The critical angledepends upon the relative indices of refraction of themedia. Critical angles for various substances, when theexternal medium is air, are listed in paragraph 2-17.

b. The total internal reflection of light can beillustrated by following the rays from a light source underwater. Water has a critical angle of 480 36 minutes,when the external medium is air. Rays of light from sucha source would be incident at various angles on thesurface separating the water and air (fig 2-35). As theangle of incidence of the light rays increases, thedeviation of the refracted rays becomes proportionatelygreater. A point is reached where an incident ray isdeviated to such an extent that it travels along thesurface of the water and does not emerge into the air.The angle formed by this incident ray and the normal isthe critical angle of the medium.

Figure 2-34. Deviation of rays by divergent lens.

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Figure 2-35. Angles of light rays from an underwater source.

c. All rays traveling in an optically dense mediumand striking the surface with an angle of incidence that isless than the critical angle of the mediums are refractedand pass into the optically lighter medium in accordancewith the laws of refraction. All such rays striking at anangle of incidence greater than the critical angle ofmediums are reflected inwards in accordance with thelaws of reflection.

d. The critical angle has a practical application inthe design of prisms used in fire-control instruments. Insuch instruments, it is quite often necessary to changethe course of the path of light. This deviation could beaccomplished by mirrors but the prisms used perform thetask more satisfactorily. When prisms are employed atangles greater than the critical angle of the substance ofwhich they are made, their reflecting surfaces do notrequire silvering, yet these surfaces appear to be silveredwhen one looks into such a prism.

e. Consider a prism with two faces at right anglesand a back or hypotenuse at a 450 angle (A, fig 2-36).An incident ray, striking one of the right-angle faces inthe direction of the perpendicular or the normal, passesinto the prism without refraction or deviation until it hitsthe boundary of the medium (back of the prism). Sincethe angle of the back is 450 and the critical angle of theglass of the prism is 420, the ray cannot pass throughthe back surface of the prism. Instead, it is totallyreflected. Inasmuch as the angle of incidence betweenthe incident ray and the normal (at the back surface) is450, the angle of reflection would be the same becausethese angles are always equal. The result is that the rayis reflected a total of 900, strikes the surface of the otherright-angle face of the prism in the direction of thenormal, and passes out of the prism without furtherdeviation.

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Figure 2-36. Total internal reflection in 90-degree prism.

f. At any boundary between different media thereis both reflection and refraction of an incident ray,excepting total reflection (para 2-7f). Figures 2-35 and 2-36, therefore, are only approximations.

g. The critical angle of any substance can be easilycalculated from the equation of Snell’s law(para 2-11. e), n =sin n

sin rIt follows that:

sin r = sin 90° = 1n n

For water,n=1. 333 (para 2-12c), sinr = 1= 0. 750187, and the critical angle

1. 333r = 48° 36 min (para 2-17a).

2-17. Critical Angles of Various Substances.a. The critical angles for various substances, when

the external medium is air, are as follows:Water ...................................................... 48° 36 minCrown glass ............................................ 41° 18 minQuartz ..................................................... 40° 22 minFlint glass ................................................ 37° 34 minDiamond.................................................. 24° 26 min

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b. The small critical angle of diamond accounts forthe brilliance of a well-cut diamond. It is due to a totalinternal reflection of light which occurs for a greatervariety of angles than in any other

substance. The light entering the diamond is totallyreflected back and forth a great number of times beforeemerging, producing bright multiple reflections.

Section IV. IMAGE FORMATION

2-18. General. In considering the principles of theformation of images by lenses, for simplicity ofexplanation it will be assumed that perfect single lensesare used.

2-19. Classification of Images.a. Virtual and Real Images. Two types of images

are produced by optical elements, virtual images and realimages.

(1) Virtual Images. A virtual image is so calledbecause it has no real existence. It exists only in themind and cannot be thrown upon a screen because it isapparent only to the eyes of the observer. A familiarexample is the virtual image formed by a mirror. Theimage of the person looking into the mirror appears to beon the other side of the mirror (fig 2-37). No trace of theimage can be seen on the back of the mirror. A planemirror produces a virtual image resulting in an opticalillusion in which the object appears to be behind themirror.

Figure 2-37. Virtual image reflected by plane mirror.

(2) Real images. A real image actually exists.It can be thrown upon a screen. The lenses of thehuman eye and the camera form real images. The realimage formed by a camera can be seen on the groundglass (fig 2-38).

Figure 2-38. Real image formed on ground glass ofcamera.

b. Erect, Inverted, Normal, and Reverted Images.When an image is reflected or refracted by opticalelements, the parts of the object may be seen as beingtransposed horizontally, vertically, or both. This isillustrated by what happens to the letter F shown in figure2-39 and by B, figure 2-36.

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Figure 2-39. Letter F as reflected by optical elements

(1) An image, regardless of size, that showsthe face of an object unchanged is said to be normal anderect (A, fig 2-39). When the object is seen ashorizontally transposed through 1800, it is termedreverted and erect (B, fig 2-49). Reversion is the effectseen in a vertical mirror where the image is reversed sothat the right side of the object becomes the left side ofthe image (fig 2-37).

(2) The image of an object with its faceunchanged but upside down in termed normal andinverted (C, fig 2-39). Inversion is the effect seen in ahorizontal mirror where the image is inverted so that thetop of the object becomes the bottom of the image. If itappears turned from left to right as well as upside down,it is said to be reverted and inverted (D, 2-39). This isthe effect seen on the ground glass of a camera(fig 2-38).

2-20. Image Reflection by Plane Mirror.a. When the image of an object is seen in a

mirror, it appears to be located at a distance behind thereflecting surface equal to the distance of the object fromthe front of the mirror. The image is erect and revertedas are all single reflections from vertical plan mirrors.

b. In a dark room, the image of a tiny point of lightas viewed in the mirror appears to the observer to belocated behind the mirror and on the other side of theroom where it actually is. The observer sees along thepath of the reflected ray to the point where the ir dent rayis reflected by the mirror. His line of sir :is extended inhis mind in a direct line through and beyond the mirror.The apparent position of the point of light in the mirror islocated directly across the room from the light sourceand at the same distance behind the mirror as the lightsource is in front of the mirror (EYE "A", fig 2-40).

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Figure 2-40. Reflection of point of light in mirror.

c. Regardless of where the observer stands, if hecan see the reflection of the point of light in the mirror, itsapparent position is unchanged. This holds true whetherhe walks forward along his line of sight without affectingthe angles of incidence and reflection of the ray or stepsto either side changing his line of sight and the angles ofincidence and reflection (EYE"B", fig 2-40). It is theobject (source of light) that is reflected and the apparentposition of its reflection is changed only when theposition of the object or of the mirror is changed.

d. Now, assume that the light source is replaced

by a cutout letter F covered with luminous paint (fig 2-41).Light from every point on the letter sends outincident rays which are reflected by the mirror. Eachincident ray and reflected ray obeys the laws of reflectionand their paths can be plotted accordingly. As the result,the entire image formed by the combination of thisinfinite number of images of individual points of light isreflected to the eye of the observer. The observer,looking along the paths of the reflected rays, sees theimage formed by the points of light. The image he seesappears to be in back of the mirror as an erect, revertedimage (fig 2-41).

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Figure 2-41. Reflection of letter F in mirror.

2-21. Image Transmission by Mirror and Prism.a. Image Transmission by Mirror. Image

transmission by mirror is image reflection put to practicaluse. The mirror is mounted so that it will transmit thelight that falls upon it to whatever point is desired. If thiscannot be accomplished with a single mirror, a secondmirror is placed to catch the reflected light from the firstmirror and transmit it again.

(1) Consider two mirrors placed at 900 anglesto one another. The light coming from the letter F fallsonto the reflecting surface of one of the mirrors(fig 2-42). Light rays from every point on the letter F arereflected by the first mirror, according to the laws ofreflection, to the second mirror, and reflected again, thistime parallel to the original paths of the light rays. Theimage will have been reflected a total of 180�.

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Figure 2-42. Image transmission by mirrors.

(2) The combination of two mirrors at 90° angledoes not invert or revert the image when reflection takesplace in the horizontal plane (the figure shows the rear ofthe object and the front of the image). If reflection takesplace in the vertical plane an inverted, reverted image isobtained.

b. Image Transmission by Prism. Imagetransmission by prism can be brought about in practicallythe same manner as an image is transmitted by oneplane mirror (B, fig 2-36) or two plane mirrors (fig 2-43).Actually, the reflecting surfaces of a prism are planemirrors. These surfaces are silvered when the angle atwhich the light strikes is less than the critical angle (para2-16) of the material from which the prism is made; thesurfaces require no reflecting agent when this angle isgreater than the critical angle.

Figure 2-43. Image transmission by prism.

c. Comparison. When mirrors are used, the imageis reflected through air; when a prism is used, the imageis reflected through glass. While being transmittedthrough glass, the paths of the rays of light are notdeviated because they travel in a medium of constantoptical density. These paths are not deviated uponentering or leaving the glass of the prism provided thelight rays strike the surface of the glass in the direction ofthe normal, at right angles to the surface.

2-22. Focal Length and Focal Plane.a. Focal Point of Convergent Lens. When light

strikes and passes through a convergent lens, the raysfrom a point of light intersect at a point on the oppositeside of the lens. The point at which the rays intersect istermed the focal point.

b. Focal Point-Location. The focal point of a lenswill vary in relation to the distance of the

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object. If a sheet of ground glass or thin paper is placedacross the back of a camera or held in back of a lens ata distance that will permit the lens to focus the images ofobjects on the glass or paper, images of all distantobjects may be brought into sharp focus at one timeregardless of their distance providing the distance isgreat -and rays of light are almost parallel. An objectvery close to the lens will be out of focus. The groundglass or paper will have to be moved backward to bringsuch an object into sharp focus; then the more distantobjects will be out of focus.

c. Point of Principal Focus. The focal point wherealmost parallel rays from a very distant object intersect iscalled the point of principal

focus (fig 2-44). The point of principal focus of a lensdoes not vary, it always remains at the same distancefrom the lens. The optical center of a lens is a point atthe center of the thickest part of a converging lens or thethinnest part of a divergent lens. It lies on a straight lineconnecting all the principal focal points of the lens (fig 2-45). Rays of light pass through the optical center withoutdeviation. The distance from the point of principal focusto the optical center of a convergent lens is the focallength of the lens. This is a fundamental concept whichmust be kept in mind throughout to understand opticaldiagrams. The thicker the lens in relation to its size orthe steeper the curves of the lens, the shorter its focallength will be.

Figure 2-44. Focal lengths of convergent lens.

Figure 2-45. Optical center of lens.

d. Focal Plane. The focal plane of the lens (fig 2-44) is a thin flat area at right angles to the central axis ofthe lens in all directions from the focal point. It is thearea that is occupied by the ground glass or paper whichmight be used to make the image visible. In a camera,the film is placed in the focal plane of the lens. The focalpoint (fig 2-46) is the image of a single point of light onthe object; the focal plane is the combined image of allthe points of light on the object. The principal focal planeof the lens is the focal plane passing through the point ofprincipal focus (fig 2-44). In a camera, when taking aphotograph of very distant objects, the film is placed inthe principal focal plane of the lens.

e. To Determine Focal Length of Convergent

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Lens. A rough determination of the focal length of aconvergent lens can be secured by holding the lens tofocus the image of some distant object on a sheet ofpaper or ground glass. When the image is clear andsharp, this will indicate that the point of principal focushas been reached. Measure the distance from theimage to the optical center of the lens.

f. Focal Length of Divergent Lens. The point ofprincipal focus and any focal points and focal

planes resulting from the nearness of the object to thelens are located on the side of the divergent lens towardthe object or light source. The point of principal focusand other focal points are located where the emergentrays would intersect on the axis if they were extendedbackward as imaginary lines toward the side of the lenson which the light strikes (fig 2-46). The distance fromthe point of principal focus to the optical center of thelens is the focal length.

Figure 2-46. Focal length of divergent lens.

2-23. Magnification and Reduction.a. Magnification is the apparent enlargement of the

image of the object to the eye of the observer. Commonexamples are the reading glass and microscope. Inafire-controlinstrument, magnification, by marking targetsappear larger, makes them appear closer.

b. Optical reduction is the apparent reduction in thesize of the image of the object to the observer. Anegative or divergent lens placed in the floor of anairplane to permit observation functions on this principle.It makes objects appear smaller than when viewed withthe unaided eye but has the advantage of increasing thefield of view. A much larger area may be scannedthrough the relatively small opening than would bepossible if the lens were not used.

c. The degree of magnification of an opticalinstrument is expressed as the power of the instrument,for example, 8-power. An 8 power (8X)

telescope will produce an image, the apparent size ofwhich is eight times as high and eight times as wide asthe image when viewed by the unaided eye.

d. Military targets can often be seen better with 10-power magnification than with 20-power, due to thechange in the field of vision which accompanies achange in power and due to varying conditions of light.For this reason, some fire-control instruments are fittedwith means for changing the power of magnification.Certain instruments provide no magnification; theirprimary purpose is to place a reticle in the field of vision.

2-24. Image Formation by Convergent Lens.a. Types of Images. Images which are formed by

convergent lenses may be either real or virtual images.They may be larger or smaller than the object, dependingupon the distance of the object from the lens. b.

Perfect Lens Assumed. Multiple or com-

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pound lenses (para 4-2) may be used to correct for colorand other faults. A theoretically perfect single lens isassumed in this paragraph.

c. Plotting the Image. To plot the formation of animage by a convergent lens, consider that every

point of an object is emitting cones of rays of light whichare incident upon the front face of the lens. Two raysfrom a point will be sufficient to indicate the action of therays and to located the image of this point (fig 2-47).

Figure 2-47. Image formed by convergent lens.

(1) Assume that one ray from a point at the topof the object will pass through the center of the lens.Since this ray will strike the lens at the center, its path isnot deviated. A second ray from this point of the objectstrikes the lens above the axis, is refracted upon enteringand upon leaving the optically denser medium, andintersects the first ray, For this second ray, we chose aray parallel to the axis. After passing through the lens, itwill converge to the point of principal focus and thus canbe easily located. The point where

the two rays intersect becomes the corresponding pointfor the image.

(2) Two similar rays from a point at the bottomof the object form a point of the image corresponding tothe bottom of the object. Every point on the object formsits point of light on the image in the same way. Lightrays from the upper part of the object form points of lighton the lower part of the image and vice versa. Theimage is transposed diametrically and symmetricallyacross the optical axis from the object resulting in in-

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version and reversion as in figure 2-47. An imageformed in this way is real.

d. Object Location and Image Size. The image ofan object located at any distance beyond two focallengths of the lens is smaller than the object. If theobject is at two focal lengths, the image is the same size.If the object is between one and two focal lengths away,the image is magnified. All of these images are inverted,reverted, and real. If the object is at one focal length, therays are parallel after refraction; the image is said to beat infinity.

e. Magnification by Convergent Lens.(1) Conjugate foci. These are two points so

related that an object at either point will be imaged by alens at the other point and will be a real image. Themathematical relationship is as follows

1 plus 1 1Object Distance Image Distance Focal Length

or 1 1 1 (fig 2-47)0d Id f

Magnification = Size of Image = Image DistanceSize of Object Object Distance

If any three of the above quantities are known, it ispossible to determine the fourth by the application ofsimple algebra. To state magnification another way, it issimply comparing the size of the image with the size ofthe object or:

Magnification= Image Size Y1 = Id orObject Size Y Od

M = Y1=f f=x1

Y Y Od-f x f

(2) Magnification by reading glass. If the objectis inside one focal length of a convergent lens, the imagewill be on the same side of the lens as the objectmagnified, normal, and erect. The image will no longerbe real but will be virtual. This is a condition which canbe illustrated easily by the use of a reading glass (figs 2-48 and 2-69). Eyepieces of all optical instrumentsprovide the eye with a virtual image. If the observerlooks at the object, for example, a portion of a printedpage, through a reading glass held close to the type, theprinted letters appear larger than they actually are. Ashe moves the glass away from the type, each letterappears larger and remains clear provided he moves hiseye backward as he moves the glass away from theprinted page. At a certain point the printed letters appearat their largest and clearest. Beyond this point theybecome blurred and then inverted. The time when thelargest and clearest view of the letters is obtained is justbefore the focal point of the lens has been reached. Therays from the object can be plotted for any point in thepreceding demonstration (fig 2-48). A ray of lightthrough the center of the lens passes through in astraight line unchanged. A ray which is originally parallelto the axis is directed towards the point of principal focus(para 2-22). The reason for the magnification of theimage is clear considering that the virtual (not real)image is located where the emergent rays wouldintersect if they were extended backward as imaginarylines. These rays form a magnified image of the objectwhich appears to be in back of the object.

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Figure 2-48. Magnification by reading glass-object within focal length.

2-25. Image Formation by Divergent Lens.

a. Type of Image. Divergent lenses can form onlyvirtual images which are erect and smaller than theobject. This holds true regardless of the distance of theobject from the lens. Divergent lenses are termednegative because they will produce only an imagesmaller than the object, they produce only virtual images,and they deviate the light outward. The virtual imagesformed by them are always on the same side of

b. Plotting the Image. Light rays from a very

distant object strike the face of a divergent lens in almostparallel rays and, after refraction, are diverted by the lensaway from the axis (fig 2-49). In looking along the pathsof the rays, the observer sees an apparent intersection ofthe rays at a point on the same side of the lens as theobject. The point where the rays appear to intersect isthe focal point. It is backward continuation of theemergent rays as imaginary lines. As the observer looksalong the paths of these rays, he sees the virtual (notreal)--image of the object located inside the intersectionof these imaginary continuation lines (fig 2-50).

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Figure 2-49. Effect of parallel rays on divergent lens.

Figure 2-50. Reduction by divergent (negative) lens.

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c. Image Size. An observer receives theimpression that the image seen on the other side of thelens is smaller than the object. This effect is known asreduction as contrasted with

magnification produced by a simple magnifier. It isuseful in some camera viewfinders, but since it does notmagnify or produce a real image its usefulness is limitedin military instruments.

Section V. COLOR

2-26. General.

a. Ordinary sunlight or any other "white" light is amixture of visible light of all wavelengths. "White" light,is, therefore, a mixture of all colors.

b. The amount of deviation of light from its pathdepends upon the color of the light as well as upon theshape and optical density of the optical element. Eachcolor of light has its own distinctive wavelength and thereis a different index of refraction for each color. Theunequal deviation of rays of light of various colors,dispersion as it is called, has an important effect on theformation of images, and, therefore, upon the design offire-control instruments.2-27. Separation of "White" Light.

a. If sunlight is passed through a slit and then

through a triangular cross-section glass prism, theemergent light is split into a number of refracted rays. Ifthese refracted rays are made to fall upon a screen, theyproduce a band of colored light called the solarspectrum. The colors of the spectrum are, on order; red,orange, yellow, green, blue, indigo, and violet. About130 distinct hues have been observed.

b. Each color of the spectrum represents lightvibrating at a different frequency or wavelength. Theshorter the wavelength, the more the rays are bent uponbeing refracted. Red light has the longest wavelength(A, fig 2-51); violet, the shortest. Red rays are bent theleast; violet, the most; the rays of other colors areinterspaced accordingly

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Figure 2-51. Dispersion.

c. Glass of various optical densities disperses lightto different degress. A prism of flint glass disperses therays of different color more than the less optically densecrown glass. A lens would also separate light because alens may be considered as an infinite number of prisms.The properties of different kinds of glass are employed toneutralize the dispersion of the rays of colored light inlenses and prisms, and to neutralize their tendencies toform different focal points when refracted through lenses(para 2-39).

d. The principle of dispersion is put to practical usein a scientific instrument known as the spectroscope.This instrument is especially designed to disperse lightcoming from an incandescent source. Since anyelement upon being

heated to a point of incandescence emits light of afrequency unique to that element, the spectrum of anincandescent solid can be used to show of what thatsolid is composed. By the use of the spectroscope,helium was discovered in the atmosphere of the sunbefore it was discovered on earth.

e. An instrument similar to the spectroscope butfitted with a camera permits the photographing ofspectra. The photographic records made by thisinstrument are called spectrographs. This instrument isalso used to measure refractive indices and is extremelyaccurate.

f. The rainbow is a natural phenomenon resultingfrom refraction and total internal reflection of sunlight inraindrops (B, fig 2-51).

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2-28. Selective Reflection and Absorption.a. Objects derive their color from the selective

reflection and selective absorption of light. If an objectreflects practically all of the light striking it, the objectappears white. If it absorbs the greatest percentage oflight, it appears black. The surfaces of most objectsabsorb light of certain frequencies and reflect light ofother frequencies. The colors of these surfaces aredetermined by the frequencies they reflect. This isknown as selective

reflection.b. An object appears to be of a color because it

absorbs light of certain wavelengths and reflects othersstriking it. In sunlight, it may appear to be of one colorbecause it reflects the light of that color and absorbs allothers. Under a single color or monochromatic light, itmight appear to be of another color or almost black,depending upon what colors and how much light itreflected.

Section VI. CHARACTERISTICS OF OPTICAL SYSTEMS

2-29. General. Military fire-control instrumentsemploying optical systems are required for a greatvariety of purposes. To cite a few, these instruments areused to see at great distances, to magnify small objects,to determine positions, to gage distance, and to aimweapons.

2-30. Military Telescopes. Necessary characteristicsof military optical instruments are usually the following aminimum of weight and bulk, the largest possible field ofview and brightness of image consistent with thenecessary magnifying power; freedom from distortion;and a combination of ruggedness with simplicity. Theinstrument must form a normal erect image of the object.In the majority of instruments, the optical system mustinclude a reticle pattern.

2-31. Magnification. Magnification of a telescopeoptical system depends both upon the objective and theeyelens or eyepiece. Although the

eyepiece magnifies the image formed by the objective, itcannot supply sharpness or detail lacking in the image.The shorter the focus of the eyepiece, the greater is themagnifying power.

2-32. Field of View.a. The true field of view of any optical instrument is

the extent (the width and height) of what can be seen atone time by looking through the instrument. The field ofview is cone-shaped; it becomes wider and higher as thedistance is increased from the objective. In the majorityof fire-control instruments the angle of the true field ofview is relatively small (para 5-25).

b. The true field is limited by the optical possibilitiesof the eyepiece(fig. 2-52). The maximum angle whichmight be imaged by the eyepiece, if it were not for anumber of limiting factors, is termed the apparent field ofview (para 5-26)

Figure 2-52. Field of view limited by optical possibilities of eyepiece.

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c. An apparent field of view of approximately 450may be considered as a practical maximum for a highlycorrected eyepiece. Thirty--five or 400 is a morecommon value and in the more simply constructedeyepieces the field may be only 250 or 300. Thisapparent field of view, divided by the power of theeyepiece, determines the maximum value of the truefield for any given magnification. For example, if aneyepiece corrected for an apparent field of 400 is usedand a telescope of 10-power is required, the maximumtrue field of the instrument would be only 4°.

d. Other factors can further limit this maximum truefield. The components of the erecting system of theinstrument might not be sufficiently large to transmit thefull useful area of the image and would decrease thefield. Some aperture in the instrument might be so smallas to restrict the field.

2-33. Brightness of Image.a. Brightness of Image and Exit Pupil. The more

light that can be brought from each point of the object tothe eye, the brighter will be the image that is formed.This characteristic is known as brightness of image. Thepoint where the image is brought to the eye is called theexit pupil of the instrument.

b. Magnification and Brightness. When one looksat very distant objects with the naked eye, almost parallelrays of light enter the pupil of the eye and form an imageof a certain size on the retina. When one uses atelescope of 2-power magnification, the light forms animage on the retina which is twice as high and twice aswide or an image that covers four times the area of theimage that would be formed by the naked eye of thesame parts of the objects (fig 2-53).

Figure 2-53. Comparison of light entering eye with 2-power telescope.

c. Loss of Light. There is always a considerableloss of light by absorption and by reflections at thesurfaces of the lenses. While this loss may be as greatas 75 percent, every effort is made to reduce the loss toa minimum (para 4-13).

d. Objective Aperture. A contributing factor inbrightness of image is an objective lens aperture (usableportion) large enough to permit the eyelens to producean emergent beam that will fill the pupil of the eye. Onthe other hand, enlarging the objective beyond this pointaffords no greater brilliance because additional light isprevented by the iris from entering the eye.

e. Exit Pupil Aperture. During the day, the pupil ofthe eye is from 1/10 to 2/10 inch in diameter. At night,the pupil may dilate until its

diameter is from 1/4 to 3/10 inch. A telescope or otherinstrument for use at night should have an exit pupilaperture (fig. 2-54) of this size. The aperture of theobjective must then be sufficiently large (1/4 to 3/10 inchtimes the magnification of the instrument) to insure thatthe pencil of light will fill the pupil of the eye. During thedaytime, the useful aperture of the objective of such aninstrument is correspondingly smaller.

f. Sacrificing Brightness of Image. Some of thetelescopes used for military fire control purposes aremade with objective smaller than that indicated by/thisformula. Brightness of image has been sacrificed toobtain a more compact instrument.

g. Aperture Ratio of Camera Lens. This is

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the ratio of focal length to lens diameter of aperture. It isalso known as the speed of a camera lens and is writtenas f/16 (for example) or f/3. 5. At f/16 the lens is beingused at an aperture (opening) of 1/16 of its focal lengthand at f/3. 5 the lens is being used at an aperture of 1/3.5 of its focal length. As the denominator increases, thefocus becomes sharper (lessened aberration) but thelight and brightness of image decrease as the aperture issmaller. The time of exposure, therefore, must belengthened proportionally. As the denominatordecreases, the lens is said to become faster (wideraperture or lens opening) and it can be used at fastershutter speeds (shorter exposure time). If it is desired tophotograph action, a fast lens must be used; otherwise,the picture will be blurred from the motion.2-34. Eye Distance or Eye Relief.

a. The exit pupil (fig. 2-54) is the term applied tothe rays of light which emerge from any opticalinstrument with a collective or convergent eyepiece andform the image that is seen by the eye. The exit pupil ofsuch an instrument can be seen as a brightly illuminateddisk floating in space if the instrument is directed towardan illuminated area and its eyepiece is held about 15inches from the eye. The position of the exit pupil can befixed on a screen of translucent paper or on a plate ofground glass. In this manner, the distance of the exitpupil from the last glass surface of the eyepiece can bemeasured. Figure 2-54. Exit pupil.

b. The full field of view of a telescope can be seenonly when the pupil of the eye of the observer is at thesame position as the exit pupil. The distance from theeyelens to the exit pupil is

termed the eye distance or eye relief (fig 2-55). The eyemust be placed at this distance from a collectiveeyepiece in order to see most effectively through theinstrument.

Figure 2-55. Eye distance.

c. If the eye distance of an instrument is so limitedthat the exit pupil cannot enter the eye pupil or if theeyelashes touch the eyelens, observation through theinstrument will become tiresome and the instrumentcannot be used most effectively. When the instrument isto be mounted on a weapon, if the eye distance is notconsiderable, recoil, of the gun will make observationdangerous.

d. In the design of the instrument, the properlocation of the exit pupil must be given most carefulconsideration. If it is too far back, the eyepiece willbecome too cumbersome in design; if it is placed tooclose, the user of the instrument will suffer discomfortand will be unable to use the instrument to the bestadvantage. Proper eye distance depends on thepurpose for which the instrument is to be used(para 5-27).

2-35. Interpupillary Distance.a. The spacing between the pupils of the eyes

make stereovision possible because each eye views anobject from a slightly different viewpoint. From the fusedimage gained from the two viewpoints, the observerreceives the impression of depth. This spacing of theeye pupils varies to a considerable degree in differentindividuals and is known as Interpupillary distance. Thisdistance is measured in millimeters by theinterpupillometer M1.

b. In the designing of instruments to be used by

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both eyes of an individual, provision must be made toadjust the spacing between the eyepieces of theinstrument to conform to the interpupillary distance ofdifferent observers. The interpupillary distance of suchinstruments generally is adjustable from 58 to 72millimeters because the eye spacing of nearly everyoneis within this range. If the interpupillary distance of anindividual is greater than 72 millimeters or less than 58millimeters, he is incapable of using instruments of thistype to the best advantage.

2-36. Focusing.

a. For perfect focusing of a monocular instrumentunder all conditions, two adjustments are necessary. Ifthe instrument has a reticle, means must be provided foradjusting the distance between the reticle and theobjective so that there will be no parallax (para 2-46).Also, the distance between the eyepiece and the reticlemust be adjusted for the eye of the observer. In abinocular instrument, it also is necessary to adjust theinstrument to conform to the interpupillary distance of theeyes of the observer.

b. If the focal length of the objective is short andthe targets in general are at a great distance, theobjective may be adjusted for a target at an infinitedistance when the instrument is assembled in the factoryand no field adjustment is provided. This is the case withsome fire-control instruments and low-power telescopes.

c. Focusing eyepieces are commonly found on fire-control instruments. This adjustment is primarilydesigned for focusing the instrument for different eyesand is referred to as the diopter

movement. The two lenses of the eyepiece are mountedin a simple tube and its distance from the reticle or focalpoint of the objective or erecting system can be adjustedby rack and pinion, by a simple draw tube or by rotatingthe entire eyepiece causing it to screw in or out.

d. On fire-control instruments a graduated scalegenerally is provided around the eyepiece. This scale iscalibrated in diopters. The diopter is the unit ofmeasurement of the converging power of lenses (para10-2). For a normal eye the scale on the eyepiece is setat zero. If a positive 2-diopter spectacle lens iscommonly worn, the eyepiece should be set at +2provided the spectacles are removed as they should be.After carefully focusing the instrument, the reading of theeyepiece should be memorized and used for futurefocusing.

e. Low-power telescopes are frequently madewithout any means for focusing. Such an instrument istermed a fixed-focus telescope. When assembled, suchan instrument is often so adjusted that light from adistant source emerging from the instrument, instead ofbeing essentially parallel, appears to diverge from a point40 to 80 inches in front of the eyelens. A telescope sofocused (approximately minus 3/4 to minus 1 diopter) ismore readily adaptable to the eye of the averageobserver than one focused so that the rays of light areessentially parallel. Because a fixed-focus instrument issimple, it can be made entirely waterproof. However, ifthe power of the telescope is greater than 3. 5 or 4, theaccommodation of the average eye is not sufficient topermit its use.

Section VII. ABERRATIONS AND OTHER OPTICAL DEFECTS

2-37. General.

a. Aberration is a lens or prism imperfectionresulting in an image that is not a true reproduction of theobject.

b. In designing an instrument, correction is usuallymade for optical defects with special attention beinggiven to the use to which the instrument is to be put.Correction is achieved by using lenses or prisms madeof two or more kinds of glass (called compound lenses orcompound prisms), and by eliminating rays which wouldbe refracted through the outer edges of lenses (calledmarginal rays) by equipping the instrument withdiaphragms (field stops) (para 2-38 e), and a suitableeyepiece.

c. There are six general types of aberrations:spherical and chromatic aberrations, astigmatism,

coma, curvature of image, and distortion. Other factorswhich may affect the operation of the optical system ofan instrument are resolving power, Newton’s rings, lightloss, and parallax.

2-38. Spherical Aberration.

a. Light rays refracted through a lens with sphericalsurfaces near its center and those refracted through theouter portion or margin do not intersect the axis at asingle point. The outer rays of a convergent lensintersect the axis closer to the lens than the more centralones (fig 2-56) and the opposite is true of a convergentlens, considering the imaginary extension of therefracted rays (fig 2-57). The result is a blurred image.This fault is common to all single lenses with sphericalsurfaces and is termed spherical aberration.

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Figure 2-56. Spherical aberration of convergent lens.

Figure 2-57. Spherical aberration of divergent lens.

b. The thickness of a lens and its focal lengthinfluence the amount of spherical aberration. Sphericalaberration is least in thin lenses of long focal length.

c. If the convergent lens were ground withconstantly flattening curves to the edge instead of beingground with each refracting surface as a true portion of asphere, the rays would be caused to cross the axis atnearly a single point and a sharp image would result.Such lenses, however, would be too difficult and costly tomanufacture and would be correct for only one distancefrom the object. Instead, two other methods are utilizedeither singly or together which satisfactorily eliminatespherical aberration (f and g below).

d. Hourglass distortion, barrel distortion, andcurvature of image all result from spherical aberration(para 2-42). Replacing lenses so they face the wrongway ’usually will introduce distortion into the system.This is of interest, therefore, to the instrument repairman.

e. In a lens system such as a complex cameraobjective or in a single lens used at a wide angle,spherical aberration can be reduced at the expense oflight intensity by using an aperture stop, field stop, ordiaphragm. The central portion of a lens is most free ofspherical aberration. Tests of a lens will show how muchof the area around the axis may be safely used to form asharp image. The practice is to mask out all rayspassing through the lens beyond this circle (A, fig 2-57).The mask used for this purpose is led an aperture stop.It is a flat ring or diaphragm of metal or other opaquematerial covering the outer portion of the lens. It stopsthe rays from entering the margin of the lens but, as itcuts down the effective size of the lens, it limits theamount of light passing through the lens. Obviously, ingeneral, it would be cheaper to manufacture a 4mallerlens rather than to make a large lens and a stop formasking out marginal rays. Diaphragms or stops haveother uses. See diaphragms or stops in paragraph 4-10.

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Figure 2-58. Spherical aberration reduced .

f. In fire-control instruments, spherical aberration iscommonly eliminated by the use of a convergent and adivergent lens cemented together to form a singleelement known as a compound lens (para 4-2). Thecompound lens approximately corrects sphericalaberration because the concave curves of the divergentlens neutralize the positive aberration of the convexcurves. The refractive power of the combination isretained by the proper choice of indices of refraction foreach of the two lenses. A lens in which spherical andother aberrations have been minimized or eliminated issaid to be aplanatic (fig 2-59).

Figure 2-59. Effect of compound lens on sphericalaberration .

g. Spherical aberration is minimized by "bending"the lens. Bending is accomplished by increasing thecurvature of one surface and decreasing that of theother. This tends to eliminate spherical aberration whileretaining the same focal length. In telescope design, it iscommon practice to minimize spherical aberration in yetanother way by placing the greater curvature of eachlens toward the parallel rays so the deviation at eachsurface is nearly equal as in B, figure 2-58. The anglesof incidence and emergence must be equal for minimumspherical aberration. In accord with this rule, telescopeobjectives always are assembled with greater curvature(the crown side) facing forward.

2-39. Chromatic (Color) Aberration.

a. White light consists of all colors. Upon beingrefracted through a prism, white light is dispersed intorays of different wavelengths forming a spectrum ofvarious colors (para 2-27). The rays of different colorsare refracted to different extents; red undergoing theleast refraction and violet the most. Inasmuch as a lensmay be thought of as being composed of an infinitenumber of prisms, this dispersion also exists where lightis refracted through a lens. This produces an opticaldefect, present in every uncorrected single lens, knownas chromatic aberration or chromatism. The violet raysfocus nearer the lens than the red rays (fig 2-60) and therays of the other colors focus at intermediate points.Thus, such a lens would have a different focal length foreach color and the image would be fringed with

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color. Similarly, spherical aberration is greater for bluerays than for red rays because the focal length. of a lensis shorter for blue than for red rays. This is due to redrays being refracted the lesser of the two. The marginalparts of a blue real image

formed by a positive lens will show compression, whilethe marginal parts of a blue virtual image viewed througha positive lens will be more distended than the red rays.

b. The aforementioned chromatic aberration, toprevent trouble in optical instruments, must be correctedby spacing between lenses and by adjusting curvatures.A, figure 2-61, illustrates the diminishing to such effectsby equalizing the deviation at the two surfaces. Likespherical aberration, chromatic aberration is corrected bymaking a compound lens of two separate lenses; onepositive (convergent) and one negative (divergent). Thepositive lens is made of crown glass while the negativelens is made of flint glass (B, fig 2-61). Crown glass ismore strongly convergent for blue rays than for red, whileflint glass is more strongly divergent for blue rays than forred. The high color dispersion of the flint glass negativelens is sufficient to compensate for the lower colordispersion of the crown glass positive lens withoutentirely neutralizing its refractive power. A compoundlens so designed is said to be achromatic. Constructionof a lens of this type is described in paragraph 4-2b.

Figure 2-61. Correction of chromatic aberration of lens.

Figure 2-60. Chromatic Aberration

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c. Spherical and chromatic aberrations usually arecorrected by the same two elements of a compoundlens. In some lenses, the negative element isplanoconcave (only one surface ground spherically). Inthis case, the adjacent surfaces are ground tocompensate for these two defects. In others, the innercurves of the lenses are calculated to eliminatechromatic aberration and the outer curves to removespherical and other types of aberration.

d. The majority of prisms employed in fire-controlequipment are used to reflect light; some are used torefract light. When light is reflected by a prism, there isno chromatic aberration because the light rays are notdivided up or dispersed. Only when light is refracted by aprism, as through a measuring wedge of a range

finder, is the light affected by chromatic aberration.e. Chromatic aberration in a refracting prism is

corrected in much the same manner that it is in a lens.Two prisms made of different kinds of glass arecemented together (fig 2-62). The prism having thelarger refracting angle and which is to be the means ofrefracting light from its normal path is made of crownglass. The prism having the smaller refracting angle ismade of heavy flint glass which has a large dispersion ofcolors. The flint prism, by reason of its large dispersion,neutralizes the dispersion caused by the crown prismwithout entirely neutralizing the deviation of the path oflight. This compound prism is known as an achromaticprism.

Figure 2-62. Correction of chromatic aberration of prisms.

2-40. Astigmatism in Lenses.a. Astigmatism is a lens aberration which makes it

impossible to get images of lines equally sharp whenthese lines run at angles to one another. This opticaldefect is found in practically all but relatively complexlenses known as anastigmats which are designed topractically eliminate this condition.

b. A perfect lens refracts rays from a point of lightto a sharply defined point of light on the image. Theuseful rays which form the image are refracted as a cone(fig 2-63). Each cross section of one of these cones iscircular in form; each successive circle becoming smalleruntil the focal point is reached.

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Figure 2-63. Proper refraction of light .

c. A lens with spherical or plane faces properlyground will not show astigmatism for points near the axisbut will show astigmatism for points laying at aconsiderable distance from the axis. The face of thelens is then presented obliquely to the incoming lightrays. Cross sections of the cone of light refracted by thelens become successively narrow ovals until theybecome a line in the

vertical focal plane; then they become broader ovals untilthey are circular; and then they again become a line inthe horizontal focal plane at right angles to the first line(fig 2-64). Between the two focal planes is an areaknown as the circle of least confusion. It is in this planethat the most satisfactory image is secured.

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Figure 2-64. Astigmatic refraction of light .

d. Astigmatism is reduced to acceptable quality bythe use of several lenses in the same manner asspherical and other aberrations. Lenses are made ofoptical glasses possessing different degrees ofrefraction, ground to different curvatures, so that theaberrations of all types cancel each other.

2-41. Coma.

a. Coma is due to unequal refracting power of thevarious zones or concentric ring surfaces of a lens forrays of light coming from a point which lies a distance offthe axis. It is caused by the rays from the various zonescoming to a focus at slightly different points so that theyare not exactly superimposed. It appears as blurring ofthe images for points off the axis.

b. The image of a point of light is formed by a coneof useful light rays refracted through a

relatively wide portion of a lens. The lens may beconsidered to be divided into concentric circular zones orrings of varying thickness. To form a sharply definedpoint of light, the rays from each zone must come to afocus at exactly the same place in the focal plane.

c. In a lens producing coma, rays of light originatingat a point located off the axis and refracted through theinner zone form a well defined image of the point. Raysrefracted through the next zone form a larger, less-defined image of the point which is offset slightly fromthe first. The image formed by each successive zone islarger, less defined, and farther removed from the initialpoint of light (fig 2-65). The displacement of thesuccessive images is in a direction toward or away fromthe center of the field of the lens.

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Figure 2-65. Coma

d. The total image of the point offset from the axismay be any of a wide variety of patterns, an egg-1, pear-,coma-, or comet-shaped blur (fig 2-66). Its generalappearance of a comet gives it the name of coma.When viewed under a microscope, a point of lightinfluenced by coma may have a very fantastic shape dueto its being affected at one time by all types ofaberrations. Inasmuch as coma causes portions ofpoints of light to overlap others, the result is blurredimages of objects in the portion of the field affected bycoma.

AR910072Figure 2-66. Appearance of coma greatly magnified

e. In producting compound lenses the severaltypes of glass and the curves of the various faces arecarefully chosen to give approximate correction for comaas well as for other aberrations. Aplantics are lensescorrected for spherical, coma and chromatic aberration.2-42. Curvature of Image and Distortion.

a. Definition of Distortion. Distortion is a form ofspherical aberration in which the relative location of theimages of different points of the object is incorrect.When imaged by a lens having this defect, a straight lineextending across the field is curved. This is a seriousdefect in instruments of high magnifying power becausethe amount of distortion present is increased inproportion to the power of the instrument.

b. Causes of Distortion. Distortion (fig. 2-67) isparticularly harmful when the object is held in closeproximity to lens and is caused by rays of light fromdifferent points of the object being refracted by dissimilarportions of a lens. When a line extends across the field,is located close to lens, and is off clear, rays from themiddle of the line strike the lens nearer its center. Theserays are refracted at a different angle from that of raysstriking near the margin of the lens, giving a curvedinstead of straight appearance to the image.

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Figure 2-67. Barrel-shaped (A and B and hourglass (C and D) distortion .

c. Forms of Distortion. If the curvature of the linesof the image is away from the center of the lens, thedistortion is termed barrel-shaped (A and B, fig 2-67). Ifthe lines curve toward the center, the distortion is calledhourglass or pincushion (C and D, fig 2-67).

d. Curvature of the Image. The field of a lens is thearea in which the image produced by the lens

is formed. This area should be flat in order to form anundistorted image. When it is not flat, but is concave orsaucer-shaped, the condition is termed curvature of theimage or curvature of the field. An ordinary readingglass plainly shows curvature of the image (fig 2-68). Infire-control equipment this defect causes distortion inportions of the field located a distance off the axis.

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Figure 2-68. Hourglass distortion in magnification by reading glass .

e. Formation of Image Curvature. Curvature of theimage is readily visualized by assuming that a cross isimaged by a lens with this defect (A, fig 2-69). Raysfrom points of light at the ends of the arms of the crossare brought into focus nearer the lens than rays from thecenter of the cross. If the

lens is focused on the enter of the cross, the ends of thearms are not _ sharp focus and vice versa. This isparticularly disadvantageous in a photographic lenswhere a flat plate or film is used. If the image is to be infocus over the entire plate, the image also should be flat.

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Figure 2-69. Curvature of image .

f. Rays and Pencils. A definition of eccentric andparaxial rays and of centric, eccentric, and paraxialpencils will clarify curvature of image in sphericalaberration. Eccentric rays are rim rays which passthrough a lens remote from its center. Paraxial rays arethose along the axis of the lens Oblique centric pencilsare cones of light which pass through the center of a lensat a considerable angle to the principal axis. Paraxialpencils are

those along the axis. Eccentric pencils are those whichpass through the lens near the rim, remote from itscenter.

g. Hourglass Distortion. Paraxial magnification of aconvergent lens (magnification through the lens near theoptical axis) is actually less than the true magnification ofthe lens. Spherical aberration thus appears as hourglassor pincushion distortion when a vertual image is viewedthrough an un

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corrected lens as in C and D, figure 2-67. Theextremities of such a virtual image will be curved awayfrom the lens so that we call this phenomenon curvatureof image as in B, figure 2-69.

h. Barrel Distortion. Eccentric rays refracted moreby a convergent lens and cross the axis closer to thelens than paraxial rays. Oblique centric pencils alsofocus closer to the lens than paraxial pencils. As the realimage is formed where the eccentric pencils and thecentric pencils come to a focus, it is obvious that such animage formed by an uncorrected convergent lens will becurved with the extremities of the image close to the lensas in A, figure 2-69. This curvature of image isespecially troublesome in wide-angle instruments. Thisimage on a screen will present barrel distortion (A and B,fig 2-67) and it will be impossible to focus all the imageclearly at one time on the screen. On a screen of thesame curvature as that of the image, no distortion or out-of-focus effect would appear. The human eye used sucha screen as in C, figure 2-69.

i. Correction for Distortion and Curvature of Image.Distortion or curvature of image is partially corrected byemploying a compound lens, consisting of oneconvergent and one divergent lens, the two havingdifferent types of distortion. In certain cases, field stopsor diaphragms may also be used to restrict the passageof undesired marginal rays. A highly corrected distortion-free eyepiece or lens system is called orthoscopic; itgives an image in correct normal proportions and

gives a flat field of view.

2-43. Resolving Power of Lens.a. When two points viewed through a lens are so

close together that they cannot be distinguished as twodistinct points, these points are not resolved. To makeseparation of such points possible, a lens of greaterresolving power is required. The measure of theresolving power of a lens is the limiting angle ofresolution which is the angle subtended at the opticalcenter of the lens by two points which are close togetherand can be just barely distinguished as two distinctseparate points.

b. It might be supposed that a point of light on theobject on being refracted through a high qualitycompound lens would form a single point of light on theimage. However, a factor termed diffraction tends toenlarge the point of light so that it merges with anotherpoint close to it.

(1) Diffraction causes the image of a point oflight to become a tiny disk of light surrounded by a seriesof concentric rings of light which rapidly fall off inintensity, a minute bull’s-eye target of light. This can bedemonstrated to the best advantage by sharply focusingthe lens on a very small single point of light. The image,if it were greatly magnified, would resemble a series ofconcentric rings (fig 2-70). The images of more detailedobjects are made up of these tiny bullseyes.

(2) Diffraction sets the final limit to thesharpness of the image formed by a lens.

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Figure 2-70. Diffraction pattern, greatly magnified .

c. In practice, the resolving power of a lensselected for a specific purpose is usually so great thatany lack of sharpness of detail is too slight to beobserved and, therefore, too slight to be objectionable inthe performance of the service for which the lens isintended.

2-44. Newton’s Rings.a. When position (convergent) and negative

(divergent) lenses of slightly unequal curvature arepressed one against the other, irregular bands orpatches of color appear between the surfaces. Thispattern is called Newton’s rings (fig 2-71) after Sir IssacNewton who directed attention to it. This condition is adefect if it occurs in a compound lens. It may be utilizedto advantage as a means for testing the accuracy ofgrinding and polishing lenses.

Figure 2-71. Newton’s rings .

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b. When testing lenses for accuracy of grinding andpolishing, a test lens having the desired curvature isused. The test lens is placed in contact with the lens tobe tested, care being taken that both surfaces areperfectly clean and dry. When the lenses have beensqueezed together, if the lens being tested is perfect, theair film between the two lenses will be of uniformthickness and the color will be uniform all over thesurface. Irregular bands or patches of color show thatthe surface of the tested lens is not perfect and point outclearly the parts of the lens needing attention.

2-45. Light Loss.a. Whenever light rays strike the surface of any

lens or prism, a certain amount of light is lost byabsorption and reflection. Light is absorbed by everyelement it strikes or travels through. The more elementsin the optical system, the more light is lost by absorptionand reflection.

b. Light loss by reflection from surfaces has beengreatly reduced by coating the surfaces of elementswhich are intended only to refract light (paras 4-13 thru4-16). The additional light transmitted by instruments inwhich various

elements have been coated produces brighter, effectiveimages.

2-46. Parallax.a. Parallax is apparent displacement of one object

with respect to another due to the observer’s change ofposition (A, fig 2-72).

b. Parallax in a telescope sight is observed as arelative displacement (apparent movement) between thereticle and the field of view. Parallax is present when thereticle is displaced from the image plane; in other words,it is present when the image is not formed at the reticlebut is located either in front of or behind the reticle as inB, figure 2-72. Parallax can be eliminated either bymoving the objective lens, thus shifting the image, or bymoving the reticle to coincide with the image. In a high-power instrument (above 4X), it is necessary to providemeans of moving the objective to eliminate parallax if theinstrument is to be used at all ranges, for example, from50 feet to infinity. The engineer’s transit has such anobjective-focusing device as do all target-type telescopicsights used on rifles.

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Figure 2-72. Parallax

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CHAPTER 3

THE HUMAN EYE

Section I. GENERAL

3-1. Introduction. All creatures endowed with sighthave eyes of some sort; some are simple and otherscomplex, some adapted to long range vision and othersto short distances. Animals and birds of the mountainsand plains, where vision is unobstructed for greatdistances, have visual ability which enables them to pickout objects too small for human perception. This abilityappears to be associated with small nerve, endings inthe retina, faster retinal response to observed motion,and better interpretation by the brain. The other extremeis represented by the short range compound eye asfound in insects and crabs. This arrangement consistsof a patchwork of individual "eyes," each of whichrecords a spot of shade or

light, and response of the whole patchwork makes arough image but is coarse and inefficient in makingvisible any details of the outline in pattern resolution.

3-2. Comparison of Eye and Camera. The humaneye, which is rather a sturdy organ of wonderful design,may be called a living automatic camera. A high-gradecamera closely resembles the eye in basic essentials.Each has a compound lens to refract light rays and toproject them to definite points by focusing. Each has adiaphragm to regulate the entering light, a sensitizedsurface to receive and record optical images, and alightproof chamber to shut out extrenous light and toprotect the receiving and recording mediums (fig 3-1).

Figure 3-1. Comparison of eye and camera .

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Section II. CONSTRUCTION

3-3. Structure of the Eye.a. The eye is an organ nearly spherical in shape,

approximately an inch in diameter which rotates throughquite a wide range in a socket in the bone structure ofthe head. Muscles control the eyes in associatedmovements to a considerable extent and in individualmovement to a lesser degree. Protection and shade arefurnished by the eyelids, lashes, brows, and by theconfiguration of bones.

b. The eye has three coats which afford it

protection, supply it with nutrition, shut out extraneouslight, and transmit visual impressions to the brain (fig 3-2). It has a refracting mechanism consisting of thecrystalline lens, cornea, aqueous humor, and vitreoushumor; together, these form a compound lens. Theshape of the lens is changed by the ciliary muscles whichserve to change the focus of the lens. The iris serves asa diaphragm to regulate the amount of incoming light

Figure 3-2. Cross section of eyeball .

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c. The clearest and most distinct vision of theresponse mechanism of the eye is centered in a smallarea called the macula or yellow spot on the inner coat orretina and in a still more sensitive smaller spot in thecenter of the macula called the fovea or fovea centralis.Sensations of light are received by tiny cones and rodsattached to nerves and distributed over the fovea,macula, and retina. These sensations are transmitted tothe brain by the optic nerve at the lower rear of theeyeball.

3-4. Three Coats or Tunics.a. The outer of the three coats or tunics of the eye

is the sclera (fig 3-2) to which are attached the sixmuscles that hold the eye in place. It is tough, white, andflexible and is the white portion of the eye normally seen.The slightly protruding tough transparent portion at thecenter front of the eye, the cornea, is part of this coat.The cornea and the transparent liquid or aqueous humorbehind it are part of the refracting mechanism of the eye.The aqueous humor is essentially a weak salt solution.

b. The middle coat, the choroid, is a deep purplelayer made up of veins and blood vessels which suppliesnourishment to the eye tissues. The coloring of thechoroid forms a dark housing and prevents external lightfrom diffusing into the eye through the walls of theeyeball.

c. The inmost coat of the eye is the retina, a highlysensitive layer of nerve fibres by means of which visualimpressions are transmitted to the brain. The retina is apart of the response mechanism. The interior of the eyeis filled with a

thin jelly-like substance, the vitreous humor, which istransparent.

d. Light entering the eye passes successivelythrough the cornea, the aqueous-humor, the crystallinelens, the vitreous humor, and falls on the retina.

3-5. Refracting Mechanism.a. The chief refracting surfaces of the eye are

those of the cornea and the crystalline lens. The corneaprovides the constant portion of the refraction. Thecrystalline lens supplies a variable degree of refractionwhich permits the eye to focus on near or distant objects.This gives to the eyes the ability to see far or nearobjects distinctly and is termed the power ofaccommodation.

b. The iris is a diaphragm which contracts anddilates to regulate the amount of light entering the eye(fig 3-3). The iris is the variously colored portion of theeye around the pupil. The pupil is the circular opening inthe center of the iris. Its size is limited by the contractionor dilation of the iris. It appears black because of thecomparative darkness of the inside of the eyeball. Thepupil varies in size from 2 to 8 millimeters, dependingupon the illumination. In intense illumination, the irissteps the pupil down to about 2 millimeters. In moderate(daytime) illumination, the opening is about 4 or 5millimeters. This is considered to be the opening formaximal acuity or best resolution. In very faint (night)illumination, the diameter of the pupil is approximately 8millimeters

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Figure 3-3. Action of iris compared with camera diaphragm .

c. The crystalline lens is transparent and issuspended by the ligaments and muscles of the ciliarybody which encircles it (A, fig 3-4). The front of the lensrests against the aqueous humor, the rear against thevitreous humor. The suspensory ligaments and muscles

of the ciliary body draw upon or release the outer edgesof the lens to change its refracting power by altering itsshape.

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Figure 3-4. Suspension and action of crystalline lens .

d. The crystalline lens is double-convex and thefront surface is flatter than the rear (fig 3-2). When theeye is relaxed (B, fig 3-4) the lens of a normal eye willfocus upon distant objects. To increase the refractivepower of the lens, for focusing the eye upon a closeobject, the surfaces are made more strongly curved,more convex by the compressive action of the tensemuscles of the ciliary body (C, fig 3-4). This process isknown as accommodation, permitting the normal adulteye to view near objects as close as 25 centimeters(about 10 inches). An object viewed at this distance issaid to be at the near point of the eye. The ability toaccommodate usually decreases as a person gets oldermaking the wearing of bifocals

necessary in most cases. Bifocals contain lenses havinga small section ground to a higher power through whichthe user looks at near objects.

e. The image formed on the retina is inverted, butwe do not see the image inverted, as there is simply acorrespondence between the retina and externaldirections.

3-6. Response Mechanism.a. The response mechanism of the eye, the area

on which images are formed and examined consists ofthe retina (fig 3-5) or thin inner coat of the eyeball whichis sensitive to light. The cause of light sensitiveness ofthis area is an almost infinite number of visual cells.They are connected by nerve fibers to the optic nerve.

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Figure 3-5. Cones and rods of retina .

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b. The light sensitive elements or visual cells are oftwo distinct types called rods and cones (fig 3-5). Therods are cylindrical-shaped and longer than the cones;the shorter cones are bulb-shaped. All are so small thatthey must be magnified to 300 times their actual size tobe visible. While the mechanism of the eye has not beenfully explained it appears that the cones provide daylightvision, where light intensities are high; the rods are moresensitive to light, and give twilight vision, where lightintensities are low. The cones give clear sharp vision forseeing small details and the distinguishing of colors; therods detect motion.

c. A slight depression in the retina on the visualaxis of the eye is called the macula or yellow spot. It isabout 2 millimeters in diameter and contains principallycones and only a few rods. At the center of the maculais the fovea or fovea centralis, where the retina is muchmore highly developed than elsewhere, s small areaabout 0.25 millimeter in diameter containing cones alone.This area of the retina, therefore, gives the sharpestdetail and the best appreciation of color. No fibers of theoptic nerve overlie it.

d. The rods are stimulated by a substance knownas visual purple to increase their sensitiveness ofsubdued illumination. This substance builds up andaccumulates during the time an eye is becomingaccustomed to darkness. An eye affected in this manneris termed a dark-adapted eye. The visual purple isbleached out by light. A different substance serves thecones in the same way to produce what is known as alight-adapted eye.

e. One small portion of the retina is insensitive toall light. This is an area known as the blind spot or opticdisk. It is where the optic nerve enters the eye and islocated slightly to one side of the macula. In mostinstances, this blind spot has no effect on vision becausethe mind has learned to ignore it.

f. The optic nerve carries visual sensations to thebrain. This nerve is really a cable of nerves throughwhich the rods and cones activated by light images sendmessages to the brain which result in what is termedsight.

g. Foveal vision is much more acute thanextrafoveal vision and the muscles controlling the eyealways involuntarily rotate the eyeball until the image ofthe object toward which the attention is directed falls onthe fovea.

h. The outer portions of the retina serve to give ageneral view of the scene and to warn of an objectapproaching within the field, while the fovea enables theobject of chief interest to be examined minutely. If theeye is directed at a particular point on a printed page onlythe words close to that point are seen distinctly.

i. An impression registered upon the retina and theoptic nerve persists, at least as a mental effect, for anappreciable time (about 1/15 sec) after the stimulus hasbeen removed. Thus, the glowing end of a whirling stickgives the impression of a streak of light. This is knownas persistence of vision. It is this effect which is the chiefaid in securing the illusion of smooth motion in movingpictures so that we are not aware of the gaps betweensuccessive pictures.

Section III. BINOCULAR (TWO-EYED) VISION STEROVISION

3-7. General.a. Binocular vision is coordinated two-eyed vision.

The two eyes of a person are normally identical and theywork together as a team. The muscles used inadaptation and accommodation of the eyes aresympathetic in action; that is, they tend to dilate,contract, and focus together. Both eyes usually meetwith the same light conditions and are turned to convergeon the same object or field of view. The two imagesreceived by the eyes are not seen separately but arefused by the brain into a single image.

b. Stereoscopic vision, stereovision, or stereopsis,as it is called, is inseparably associated with binocularvision because it is the result of seeing with both eyes.Stereovision is the power of depth perception; the abilityto see in three dimensions. This power is due to thespacing

between the pupils of the eyes which enables the eyes tosee objects from slightly different angles. Eyes normallyare about 62 millimeters apart. This distance betweenthe eyes is called the interpupillary distance and isabbreviated "ipd".

c. The ability to record with each eye a slightlydifferent picture or image of the same object oftenenables the observer to see more of one side of a givenobject with one eye than with the other. Together, thetwo eyes see farther around the object than one eye cansee. They get a better impression of the shape of theobject, its depth, and its position with relation to otherobjects. This ability is increased through the use ofcertain firecontrol instruments which increase the virtualdistance between the pupils of the eye of the observer byoptical means.

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3-8.Field of View.a. What may be seen by the eyes rotated in their

sockets without movement of the head is termed the fieldof view; what may be seen without movement of the eyeis termed the field of fixation. The field of view of theeyes is a horizontal extent of about 1600 and a verticalextent of about 70°. In comparison, the averagetelescope has a field of view of about 10°.

b. The field of distant vision of immobile eyes isextremely limited, distinct vision being limited to only avery small central portion of the retina. Portions of thefield focused on the remainder of the retina are seenindistinctly and serve as a finder to locate objects ofinterest. When an object

of interest is located, the eye turns in its socket until theimage falls upon the central portion, the fovea andmacula, and distinct vision is secured.

c. The field of vision of both eyes includes portionswhich are viewed by the right eye, the left eye, and botheyes (fig 3-6). The binocular field exists only in thatportion of the field of view where the field of the separateeyes overlap. This explains why stereovision or depthperception is appreciated only in the center of the field ofview, it is not apparent, except by association, in theoutward portions of the field. The extreme outer areas ofthe field of view serve in the perception of motion, light,and shadow and form in two dimensions.

Figure 3-6. Field of view of the eyes .

3-9. Principles of Stereovision.a. In a military sense, stereovision implies the

ability to recognize the existence of differences in rangeto objects by visual means only. It is an ability that canbe developed by training and practice. Certain firecontrol instruments are designed to greatly increase thenormal stereovision possessed by the individual.

b. There are various means requiring the use of butone eye which can be used to determine which of twoobjects is the more distant.

(1) When one object is known to be larger thananother and both appear to be the same size, the largerobject is known to be more distant.

(2) If an object partly conceals another, thesecond is recognized as being the farther away.

(3) Differences in the amount of atmosphere

(haze), light, and shadow; the difference in focus(accommodation of the eye) required for more than oneobject in the field of view; and the relative apparentmovement of distant objects as compared with that ofnear ones when the head is moved from left to right, allinfluence the estimate of distance.

c. None of these means of depth perception are ofvalue if its contributing factors are lacking. This isdifficult to realize completely in any practical case,however, because one or more of these factors arepresent in almost all commonplace observations and aresubconsciously used by persons with two-eyed visionwhenever possible to augment their stereovision.Binocular vision can be investigated by placing two verysmall objects on a large table and then scanning themwith the eyes

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placed at the edge of the table. To gage their relativedistances, the use of both eyes and the employment ofstereovision will be required.

d. A simple demonstration of stereoscopic visioncan be made by looking at a near object such as a smallcube. The observer will see a different view with eacheye. The left eye will see the front and left side of thecube (A, fig 3-7). The

right eye will see the front and right side (B3, fig 3-7).The brain fuses these two separate pictures into a singleimage giving the impression of depth. The observersees the front, both sides, and forms a conception of thedepth of the cube, and in this way pictures the cube inthree dimensions (C, fig 3-7).

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Figure 3-7. Cube is seen by left eye, right eye, and both eyes.

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e. In much the same manner when two objects areobserved simultaneously, stereovision enables theobserver to judge the relative distance of one object fromthe other, in the direction away from the observer, ordepth. This can be done with considerable accuracywithout utilizing other characteristics which suggestnearness of distance.

f. The ability to distinguish the relative positions oftwo objects stereoscopically depends upon theinterpupillary distance of the observer’s eyes, the

distance of the objects from the observer, and theirdistance from each other (A, fig 3-8). Other factors ofdepth perception being equal, the wider the spacingbetween the pupils of the eyes, the better appreciation ofdepth, one should be able to secure by stereovision.The farther one object is from the observer, the fartheraway the second object must be from the first, if theobserver is to distinguish their relative positionsstereoscopically.

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Figure 3-8. Judging distance .

3-12

g. When a person looks at two objects andattempts to determine which is farther away, the lines ofsight from both his eyes converge or come together toform angles on both of these objects. The anglesformed by the converging lines of sight are the angles ofconvergence (B, fig 3-8). If the angles of convergence toboth objects appear to be identical, there is noimpression of one being farther away than the other. Ifthere is a difference

TM 9-258in the angles of convergence to the two objects, one ofthe objects will appear more distant.

h. The difference between angles ofconvergence may be slight but the brain possesses thefaculty of distinguishing between them. The ability to seestereoscopically is dependent upon the ability to discernthe difference between angles of convergence(A, fig 3-9).

Figure 3-9. Angular discernible difference.3-13

TM 9-258i. Angles of convergence become smaller and

the difference between them becomes more difficult todiscern as the objects are farther removed from anobserver or as their distance from one another isdecreased. This difference is known as the discernibledifference of convergence angles (B, fig 3-9) and ismeasured in fractions of minutes and seconds of arc.This provides a means of gaging the ability of observersto see stereoscopically.

j. The sense of depth perception orstereoscopic vision for the unaided eye is effective, atmost to 500 yards. In using a binocular or range finder,the lines of sight for the two eyes are further separatedthus increasing stereoscopic vision.

k. Stereoacuity (or stereopsis) is sharpness ofsight in three dimensions. It is the ability to gagedistance by perception of the smallest discernibledifference of convergence angles. This ability may

be developed by practice and training which aidsrecognition. The smallest difference betweenconvergence anges that an observer is able todistinguish is known as his minimum discernibledifference of convergence angles.

l. The minimum difference that can bediscerned between two angles of convergence dependsupon the vision of the individual observer, his state oftraining, and other general conditions such as visibility. Awell-trained observer should consistently discern anaverage difference of 12 seconds of arc and at times,under excellent observing conditions, this difference maybe reduced to as low as 4 seconds of arc for a series ofobservations.

The average untrained observer should be able todistinguish a minimum difference of 30 seconds of arcbetween two angles of convergence under normalconditions of visibility.

Section IV. DEFECTS AND LIMITATIONS OF VISION3-10. General Eye Defects.

a. Classification of Eye Defects. Common eyedefects fall into two classifications: those pertaining tosight itself and those which are the result of musculardisorder. Names ending in "ic" signify the condition while"ia" signifies the name of the disorder. Likewise, the"tropias" derived from the Greek "op" meaning sight,signify optical or sight abnormalities while the "phorias,"from the Greek "phora" meaning movement, signifymuscular disorders. The normal eye is said to beemmetropic; the person enjoys emmetropia. Theabnormal eye is ametropic; the person suffers fromametropia.

b. Common Eye Defects. The common eyedefects are listed below. Explanation may be found inthe glossary.

Nearsightedness (myopia) Farsightedness (hypermetropia or hyperopia)

Old-sightedness (presbyopia)AstigmatismColorblindness (dichromatism)Night blindnessMuscular imbalance (heterophoria)Double vision (diplopia)Squint, cross-eye, wall eye (strabismus)c. Nearsightedness and Farsightedness. The

size of the eyeball, the curvature of the cornea, and thefocal length of the compound elements are not alwaysmatched. If the lens is too convex or the retina too farback, the eye is nearsighted or myopic. This condition iscorrected by negative or digerging spectacles (fig 3-10).If the lens focal length is too long or the retina too close,the eye is farsighted or hypermetropic. This condition iscorrected by positive of converging spectacles (fig 3-11).

Figure 3-10. Nearsightedness (myopia)

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Figure 3-11. Farsightedness (hypermetropia)d. Astigmatism of the Eyes. This condition

occurs when at least one refracting surface is notspherical but is somewhat cylindrical (curvature notsymmetrical). In this case the image of a point source isnot a joint image but a short line image as in A, figure B-5. Such line images will form from sources on the opticalaxis. Thus, visual astigmatism is different from thatencountered in optical instruments (where astigmatismof a properly centered spherical lens system is zero onthe axis). This condition is corrected by cylindrical ortoroidal (toric) spectacles.

3-11. Visual Limitations.

a. Apparent Size (A, fig 3-12). This is the basisof all magnification (para 2-4 e (1)). It is measured bythe angle the object subtends at the point of

observation. This is not to be confused with actual size.A plane flying away from one will shrink in apparent sizeuntil it is only a speck in the sky. Obviously, it does notbecome any smaller but, as it recedes, light rays fromthe wingtips make a progressively smaller angle until theeye no longer can separate them and the entire planeappears as a single point. Apparent size then isinversely proportional to distance and is expressedmathematically as follows:Apparent Size =Size of Object

Distance to ObjectWhen two adjacent objects become so small in apparentsize that any further reduction in size results in failure ofthe eye to separate them, the angle of resolution of theeye has been reached.

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Figure 3-12. Visual Limitations

b. Resolving Power of the Eye. The retina is amass of nerve endings. Their individual diameters(between 0.0015 millimeter and 0.0054 millimeter)determines the resolving power of the particular eye.The size of these nerve endings vary in differentindividuals. These nerve endings may be compared to.the sensitive particles in a photographic film. Although aperfect image may be formed in the retina, it will not beperceived as such, for the distance between twoadjacent image points must be greater than the distancebetween rods or cones (nerve ends) for perception.

c. Acute Vision. Acute vision is limited to thearea of the fovea centralis, a spot (about 0.25 millimeterin diameter) on the macula which lies on the retina at thevisual axis. Here are cone nerve endings alone insteadof the longer rods with a diameter of approximately 0.002millimeter. This area of acute vision covers a true field ofless than 1° of arc (ordinarily) or a circle approximately3.5 millimeters in diameter at 25 centimeters distancefrom the eye. Least angular separation between any twodiscernible points in this field of acute vision is normallyone minute of arc (B, fig 3-12). Coincident readings, ason a vernier or micrometer, can be read closer asangular displacement of two

lines can be distinguished by adjacent cone endingscloser than displacement of points. Some persons canread accurately angular measurement (between twodisplaced lines) as small as 10 seconds of arc.

d. Measure of Visual Acuity. Visual acuity iskeeness of sight. It is the ability to see near and distantobjects clearly in order to compare minute details and todiscriminate between them. Visual acuity commonly ismeasured by viewing a standard letter of one-minutedetails, such as the following example: A person able toview the letter E composed of lines one minute of angleof width at the standard distance of 20 feet is said topossess 20/20 vision. In case a letter is used at 20 feetwhich should normally be identified at 40 feet, vision issaid to be 20/40. Some eyes, on the other hand, havebetter than average resolving power and can perceiveletters having less than one-minute details. An exampleof this is 20/10 vision, which it the ability to discern at 20feet what the normal eye can appreciate at 10 feet. Awhole series of letters have been designed (subtendingone-minute details at their respective distances) for useat the standard distance, so that the testing can be doneat a fixed distance rather than at various distances.

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TM 9-258Section V. OPTICAL INSTRUMENTS AND THE EYES

3-12. General. Optical fire-control instruments may beplaced in two general classifications: monocular, for theuse of one eye and binocular, for the use of both eyes.Both types require certain adjustments to accommodatethem to the eye as the use of any optical instrumentsaffects the functioning of the eye itself.

3-13. Accomodation of Optical Instruments to theEyes.

a. Conditions for Clear Vision. In order that theeye may function properly in the use of a monocularinstrument, the instrument must be focused to permitlight entering the instrument from an object to form adistinct image on the retina without undue effort on thepart of the eyecontrolling muscles of the observer. Theexit pupil (rear opening of the eyepiece) must besufficiently large to let the maximum amount of lightenter the pupil of the eye. Stray light must be kept out ofthe eye. In addition, the use of a binocular instrumentdemands that the two optical systems of the unit beproperly alined with each other and conform to theinterpupillary distance of the individual.

b. Focusing. Precise focusing of theinstrument permits light from the object to form a distinctimage on the retina. Focusing changes the position ofthe eyepiece with relation to the focal plane of theobjective and the angles at which the light rays arebrought to a focus. The eyepieces of focusing-typetelescopes generally are designed to accommodate therefracting qualities of the eyes of individual observers. Ifthe instrument is used consistently by an individual, theeyepiece setting can be memorized to expedite thefocusing of the instrument. Low-power telescopes havea wider range of accommodation without adjustmentsthan high-power telescopes. Telescopes in which themagnifying power is 4X or less have a sufficiently widerange of accommodation so that a single focus settingwill be satisfactory inasmuch as the eye correction is notextremely large. They are generally constructed with afixed-focus eyepiece which cannot be adjusted duringoperation. They are called fixed-focus telescopes, andusually have a minus 3/4 to minus 1 diopter setting (para10-2).

c. Use of Spectacles. If the proper spectaclesare worn, the corrected eyes will focus at the normalsetting of the instrument. However, the use ofspectacles prevents the eyes from coming up properly tothe eyeshield of the instrument and the eye may beplaced so far from the eyepiece as to restrict the field ofview. In spite of these con-

siderations, when the eyes are seriously affected byastigmatism, the observer should not remove hisspectacles as no compensating adjusting can besecured by focusing the eyepiece of the instrument.

d. Eyeshields. The proper fit and use of theeyeshields should be such that it excludes stray lightfrom the eyes. This is particularly important in nightobservation and under conditions of poor illumination sothat the pupil may dilate as much as possible. If amonocular instrument is used, the eye not is use shouldalso be shielded from light; if it receives too much light,the pupil of the eye in use will contract sympathetically.Rubber shields at the eyepiece make the most effectiveseal. Any light used to illuminate the reticle must be heldto a bare minimum.

e. Interpupillary Adjustment. A binocularinstrument must be adjusted to the eye spacing orinterpupillary distance of the observer. If this is not done,the lines of sight of both eyes cannot traverse the mosteffective optical paths of the instrument; the observer willnot have full binocular vision, nor view the most distinctimages, nor be able to seal out unwanted light from theeyepieces. Practically all instruments of this type areequipped with means for interpupillary adjustment. Theyare marked with calibrations for such adjustment. Theobserver should determine his interpupillary distance(para 2-35), memorize it, and adjust any such instrumenthe may use to conform to the spacing between the pupilsof his own eyes.

3-14. Eye Tension or Fatiguea. Blinking of the eyes is an automatic process

resulting from eye tension or fatigue. It will occur in theuse of optical instruments because the eyes cannotfocus steadily very long without relaxing. It is muscularrather than retinal, and is least apparent when the eye isrelaxed as when accommodated for distant objects. Forthis reason, in focusing an instrument, it should befocused for distance and the first distinct focus settingshould be used.

b. Fatigue of the eye muscles will beexperienced after comparatively short periods ofcontinuous observation. This fatigue usually is greaterwith low illumination. Inasmuch as the eyes quicklyrecuperate, frequent rest periods are advisable.

c. A particular type of fatigue results from theuse of binocular instruments if not set at properinterpupillary distance. This is due to the fact that botheyes involuntarily adjust themselves so that a

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TM 9-258single image is formed when the image is focused on themacula of each eye where best vision is obtained. Underordinary conditions this is done when viewing a distantobject. In using an instrument, images may be atdifferent points and

the eyes are placed in a condition. of forced equilibriumby the expenditure of an amount of nervous andmuscular force resulting in rapid fatigue.

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TM 9-258CHAPTER 4

OPTICAL COMPONENTS, COATED OPTICS, AND

CONSTRUCTION FEATURES

Section I. OPTICAL COMPONENTS4-1. Optical Glass.

a. Types. Two of the most important types ofoptical glass are known as crown glass and flint glass.Crown is an alkali-lime glass. Boro-silicate crown glasscontains salts derived from boric and silicic acids. Flintglass contains lead. The refractive indices of thesetypes of glass are listed in paragraphs 2-12c and 5-31a.

b. Characteristics. The characteristicsaffecting the values of all types of optical glass may bedivided into the purely optical properties, which directlyinfluence the light in passing throught the glass, and thegeneral physical qualities. The purely optical propertiesare constant density (homogeneity), transparencyrefraction, dispersion, and freedom from color anddefects. The general features of optical glass arechemical stability, mechanical hardness, and freedomfrom internal strain.

(1) Constant density or homogeneity is the mostimportant property. The uniformity of the index ofrefraction of a given sample of glass is dependent uponits constant density.

(2) The greater the transparency, the less lightis absorbed and the more light will pass through.

(3) The refractive and dispersive qualities aredependent upon the type of glass. Crown glass hasapproximately half of the dispersive power (ability tospread the colors of so-called white light) of flint glass.

(4) Freedom from color tint, while preferable, isnot as essential as the other requirements. It is obtainedby careful selection of raw materials which must be ofthe greatest possible purity.

(5) Types of defects are known as stones,bubbles, seeds, and striae (layers). Stones are due tosolid material that accidentally may get into the moltenglass at the time of manufacture. Bubbles are airpockets in the glass. Seeds are very minute bubbles.Striae are wavy bands which appear to be of differentdensity and color than the surrounding medium.

(6) Chemical stability and mechanical hardnessdetermine the resistance of the finished optical elementto handling and contact with atmospheric moisture.

(7) Freedom from internal strain depends uponannealing or very slow cooling of the glass inmanufacture. Any large amount of internal strain maycause the glass to break during grinding or if subjectedto shock at any time after grinding.

c. Manufacture. Although an understanding ofthe manufacture of optical glass is not essential to aknowledge of elementary optics, some appreciation ofthe many processes involved will show why fine opticalglass is considered a critical material. The finely dividedmaterials are very thoroughly mixed, usually by hand.This mixture is melted in small charges, small quantitiesbeing fed to the melting pot at intervals regulated by themelting time of the charge previously introduced. Eachcharge is brought to a liquid state before the next chargeis placed in the pot. The next operation, the finingprocess, consists of holding the molten glass mixture ata high temperature, sometimes as long as 30 hours, todrive off the bubbles contained in the liquid. To securehomogeneity and prevent striae, the molten glass is thenstirred during the gradual cooling of the mass until it is sostiff that the stirrer cannot be moved. The mass is stirredfrom 4 to 20 hours, depending on the glass and the sizeof the pot. As soon as the stirring ceases, the pot iswithdrawn from the furnace and allowed to cool to aboutthe annealing temperature. It is then placed in anannealing kiln at a temperature of from 4000 to 5000 C.and slowly cooled to ordinary temperature. Annealingtakes from 1 to 2 weeks, according to the size of thebatch. When cooled, the pot is withdrawn from theannealing kiln and broken away from the glass.

4-2. Lenses.a. Single Lenses. Lenses are optical elements

with polished faces, one plane and one spherical or

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both spherical. Lenses deceive the eyes by bending raysof light so that, depending on the type of lens, theyappear to come from closer and larger objects or fromfarther and smaller objects. Lenses are divided into twogeneral classes (fig 4-1). One

class forms real images and lenses in this class aretermed convex, convergent, positive, or collective lenses.The other class can form by itself only virtual images andlenses in this class are termed concave, divergent,negative, or dispersive lenses.

Figure 4-1. Types of simple lenses.(1) Convex lenses. All convex or converging,

positive lenses are thicker in the center than at the edgesand will converge light from sources and objects. Bothfaces of a convex lens may be convex, one surface maybe convex while the other is flat (termed plano or plane),or one face may convex while the other is concave.

(a) A double-convex lens (fig 4-1) is one inwhich both surfaces have convex curvature. Bothsurfaces contribute to the converging power of the lens.The greater the convexity of the surfaces, the shorter thefocal distance.

(b) A planoconvex lens (fig 4-1) has a planesurface and a convex surface. The plane surface doesnot contribute to the converging power of the lens.

(c) The convexo-concave or, meniscusconverging lens (fig 4-1) possesses both a convex and aconcave surface with both centers of curvature on thesame side of lens. The more powerful convex curvemakes this a positive lens despite the fact that theconcave surface tends to diverge the light, thussubtracting from the converging power of the lens.Meniscus lenses are mainly used for

spectacles because they permit undistorted visionthrough their margins ’as the eyes are rotated in theirsockers.

(2) Concave Lenses. Concave or diverging,negative lenses (fig 4-1) are thinner in the center than atthe edges and will diverge light from sources andobjects. Both surfaces of a divergent lens may have aconcave curvature (doubleconcave); one surface may beconcave and the other plane (planoconcave); or one facemay be concave while the other is convex (concavo-convex or divergent meniscus), with both centers ofcurvature on the same side of lens.

(3) Cylindrical lenses. Cylindrical lenses areground with a cylindrical surface instead of a sphericalone; they are either positive or converging, or else theyare negative or diverging. Their use is very limited. Theyare used in some of the coincidence range finders.

b. Compound Lenses. As an optically perfectsingle lens cannot be produced, two, three, or morelenses ground from different types of optical glass arefrequently combined as a unit to cancel

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aberrations or defects which are present in the singlelens (paras 2-37 thru 2-42).

(1) The refractive power of a compound lens isless than that of the convex lens alone. For example, if adouble-convex lens of crown glass is combined with aplanoconcave lens of flint glass, the latter would havelittle more than half the

refraction power of the former but sufficient dispersivepower to neutralize the dispersion. The result would bethat light passing through this compound lens would bebrought to a practical focus at a point that would beabout double the distance of the point of principal focusof the crown lens alone (fig 4-2).

Figure 4-2. Comparative focal lengths of elements in compound lens.(2) The elements are frequently cemented

together with their optical axes in alinement. Two lensesmay be cemented together as a doublet or three may becemented together as a triplet, or each lens of the unitmay be mounted separately as a dialyte (fig 4-3). In thedialyte compound lens, the inner surfaces of thedissimilar faces of the two lenses cannot be cementedtogether inasmuch as they are ground to differentcurvatures in order to correct for aberrations; the twolenses are separated by a thin ring spacer and aresecured in a threaded cell or a tube with burnishededges. The cementing of the contact surfaces, groundto the same curvature, generally is considered desirablebecause it helps to maintain the two elements inalinement under sharp blows, it aids cleanliness, and itdecreases the loss of light through reflection

at the two surfaces in contact, if the cement used, suchas thermosetting cement (para 2-12c), is a substancehaving approximately the same index of refraction asglass.

4-3. Objectives.a. General. The lens nearest the object in any

optical system of the refracting type is called theobjective. Its function is to gather as much light aspossible from the object and form a real image of thatobject. Objectives lenses in most optical instrumentsform real images (except Galilean telescope).

b. Construction. The majority of objectives areconstructed of two elements, a double-convexconverging lens of crown glass and a planoconcave flintlens (A, fig 4-3).

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Figure 4-3. Types of objectives.(1) When the elements of the objective are of

large diameter or when the faces of the elements are ofdifferent curvature, the elements are not cemented butare held in their correct relative positions in a cell byseparators and a retaining ring (B, fig 4-3 and 4-35).This construction permits giving the inner surfaces of thetwo elements different values and allows greaterfreedom in the correction of aberrations. An objective ofthis type is often called a dialyte or Gauss objective.

(2) Certain objectives are composed of threeelements securely cemented together (C, fig 4-3) or withtwo of the elements cemented and one mountedseparately, or with all three elements mountedseparately. Such objectives afford a total of six surfacesupon which the designer can secure the best possiblecorrection for aberrations.

c. Relation of Objective to Optical System.(1) The size of the objective affects the amount

of magnification that can be used in the instrument. Onlyso much light can pass through a given objective. If themagnification were increased, that amount of light wouldbe distributed over a larger image area and a dimmerimage would result. The effective size to which an image

may be magnified is, therefore, governed to a greatextent by the size of the objective. It is likewisegoverned by the limits set by distortion due to diffraction(para 2-43b).

(2) Increasing the size of the objective beyond acertain point does not improve the brightness of theimage appreciably because of the restriction imposed bythe size of the pupil of the eye.

(3) Some fire-control instruments, notably tanktelescopes, have objectives which are smaller than theeyelenses of the instruments. Most instruments of thisnature, however, have low magnification. Otherssacrifice a certain amount of the light-gathering qualitiesof the larger objective for compactness of instrument.

4-4. Eyepieces.a. General. The eyepiece may be considered

a magnifying glass which enlarges the image producedby the objective (fig 4-4). The eyepieces of modern fire-control instruments generally consist of two or threelenses; one, two, or all three of which may be compoundlenses.

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Figure 4-4. Typical light paths through field lens and eyelens of eyepieces.b. Construction. The lens of the eyepiece

nearest the eye is called the eyelens. The eyelens doesthe actual magnifying of the image and can affect thequality of the image as seen by the eye. The lens in theeyepiece nearest the objective is called the field lens.The field lens gathers the light from the objective andconverges it into the eyelens. If it were not for the fieldlens, much of the marginal light gathered by the objectivewould not be brought into the field of the eyelens (A andB, fig 4-4).

c. Relation of Eyepiece to Optical System. Theobjective takes nearly parallel rays of light from a distantobject and brings them to a focus turning them intoangular rays. The eyepiece takes these rays, which arediverging after having been brought to a focus, anddirects them into the eye.

d. Magnification. Magnification by the eyelensor eyelenses may be fixed of variable, or there may beno magnification. Some instruments are provided withchangeable eyepieces to secure different degrees ofmagnification. In other instruments, the relative positionsof lenses not in the eyepiece are changed to effectchanges in magnification.

e. Types of Oculars or Eyepieces. Types ofeyepieces used in optical fire-control instruments arediscussed in the following subparagraphs. Theyrepresent general types rather than the specificeyepieces of instruments inasmuch as a specific type ofeyepiece generally is modified to make it best suited fora particular instrument.

f. Kellner. The Kellner eyepiece (achromatizedRamsden, g below) consists of two lenses, field lens andeyelens; the eyelens is corrected for color and is adoublet or compound lens with the crown forward andthe flat flint facing the eye. For full chromatic correction,image lies in plane surface of field lens, but to use withreticle is positioned forward of field len as illustrated in A,figure 4-5. In this case, some color correction issacrificed. To reduce aberration a dense barium crownand light flint are used in the eyelens. This ocular givesan achromatic and orthoscopic field that in some modelsis as large as 500. Its most serious disadvantage is thepronounced ghost resulting from light reflectedsuccessively from the inner and outer surfaces of thefield lens. This system is common in prism binoculars.

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Figure 4-5. Kellner and Ramsden eyepieces.

4-6

TM 9-258g. Ramsden. The Ramsden eyepiece consists

of two planoconvex lenses made of ordinary crown glassof equal focal lengths separated by a distance equal totwo-thirds of the numerical value of the focal length ofeither. The reticle is located in front of the field lens atone-quarter of its focal length as illustrated in B, figure 4-5. The field lens thus contributes to the magnificationand quality of the image. Dirt on its principal plane is notin focus and, therefore, not visible. The convex surfaceof each lens faces the other. This eyepiece has thedisadvantage of considerable lateral color which can bedefined as a difference in image size for each color. It iswidely used with reticles, however. This type of ocularcan be used as a magnifier and it is therefore called apositive ocular.

h. Huygenian. This eyepiece employs acollective or field lens moved away from the eyelens sothat the real image lies between the two lenses. TheHuygenian eyepiece (A, fig 4-6) is not suitable for usewith reticles in telescopes as distortion of the reticlewould result since the individual components are notcorrected for aberration. It is sometimes used howeverwith a small reticle in the center of the field in somemicroscopes and it is useful in observation instruments.Eyepiece focal length usually used is over 1 inch forsufficient eye relief. Both elements are normally of thesame type of crown glass with convex surfaces facingforward. This type of ocular is of negative type incontradistinction to the Ramsden or positive type (gabove).

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Figure 4-6. Huygenian and symmetrical eyepieces

4-8

TM 9-258i. Symmetrical. This eyepiece employs two

doublet (compound) double convex lenses, with thegreater curvature (crown side) of each lens facing that ofthe other. The symmetrical eyepiece illustrated in B,figure 4-5, provides long eye relief (related to low powerand large exit pupil). It has a larger aperture than aKellner of the same focal length providing a rather widefield. It is used commonly in rifle scopes and gunsightsrequiring long eye relief between the eye of the gunnerand the eyepiece when the weapon recoils, and lowpower with a large field.

j. Erfle. The Erfle eyepiece illustrated in A,figure 4-7 employs three elements. One is a concavo-convex doublet field lens. Another is a symmetricalcollective center lens which adds to the power of theeyepiece by shortening the focal length and improves theillumination of the outer

portion of the field providing a more uniformly.illuminated image. The third is a double convex doubleteyelens with a slight curvature facing the eye and thegreater curvature facing the collective lens and theconvex surface of the field lens. The periscope usingthis eyepiece is the battery commander’s periscope.

k. Orthoscopic. This eyepiece is illustrated inB, figure 4-7. It employs a planoconvex triplet field lensand a single planoconvex eyelens with the curvedsurface of the field lens facing the curved surface of theeyelens. It is free of distortion and is useful in high-power telescopes because it gives a wide field and highmagnification with sufficient eye relief. It is useful inrange finders to permit use of any part of the field. It isso named because of its freedom from distortion.

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Figure 4-7. Erfle and orthoscopic eyepieces.

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l. French. This eyepiece is illustrated in A,figure 4-8. It employs elements substantially resemblingthose of the orthoscopic eyepiece in the reverse order. Itwas generally indicated on French drawings furnished.the United States of America during World War I.

Figure 4-8. French, Plossl, and variable magnificationeyepieces.

m. Plossl. This eyepiece employing anachromatic doublet for each lens is illustrated in B, figure4-8. The converging components of the doublets faceeach other.

n. Eyepiece With Variable Magnification.These eyepieces generally employ two eyelenses inaddition to the field lens. The forward eyelens is calledthe first eyelens and the eyelens nearest the

eye, the second eyelens (C, fig 4-8). Variablemagnification is obtained by changing the distancesbetween elements of the eyepiece and the erectingsystem of the telescope. A collective lens in the body ofthe instrument is often one of the elements of the opticalsystem of such an instrument.

4-5. Prisms.

a. General.(1) A prism, unlike a lens, is bounded by plane

surfaces and can be designed to deviate, displace, andreflect light in numerous ways. The introductions ofprisms into optical instruments permits design variationsotherwise impossible.

(2) In fire-control instruments, it frequently isdesirable to bend the rays of light through an angle inorder to make a shorter, more compact instrument, tobring the eyepiece into a more convenient position or toerect an image. Plane mirrors are sometimes used tochange the angles of the light rays but the silveredsurfaces tend to tarnish and cause a loss of light whichgrows more serious as the instrument becomes older. Aprism used for the same purpose can be mounted in asimpler and more permanent mount, the angles of itssurfaces cannot be disturbed, and it can produce morenumerous reflection paths than would be practical withmirrors.

(3) Optical prisms are blocks of glass of manyshapes especially designed and ground to permit themto perform the various functions. They are used singly orin pairs to change the direction of light from a fewseconds of arc (measuring wedges) to as much as 3600(Porro prism system).

(4) When the incident rays strike the reflectingsurface of a prism at an angle greater than the criticalangle (para 2-16) of the substance of which the prism iscomposed, the reflecting surface does not need to besilvered. This surface must be silvered, to insure totalreflection, if the incident rays strike it at an angle lessthan the critical angle.

b. Right-Angle Reflecting Prism. Dependingupon which reflecting surface is struck by the incidentray, the right-angle reflecting prism will bend the lightrays through an angle of 900 or 1800. The manner inwhich this prism reflects rays through an angle of 900 isillustrated and described in paragraph 2-16e (A and B, fig2-36) and through an angle of 1800 in paragraph 2-21a(1) (fig 2-43). When used to reflect rays through anangle of 900, the rays are reflected only once with theresult that the image is reverted with reflection in ahorizontal plane or inverted with the reflection in avertical plane. When used to reflect rays through anangle of 180°, the rays are reflected twice and a

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normal erect image is produced with reflection in ahorizontal plane while inverted, reverted image isproduced with reflection in a vertical plane.

c. Porro Prism System.(1) Each element of the Porro prism system is a

right-angle reflecting prism placed to bend rays of lightthrough an angle of 1800. One element is arranged tobend the rays horizontally and the other to bend the raysveertically; one element reflects the rays into the other(fig 4-9). With one prism in horizontal position and theother vertical, the Porro prism produces an inverted,reverted image. The Porro prism system is used as theerecting system in many fire-control instruments; it iscommonly used in binoculars. The sharp corners ofthese prisms are removed making the ends round. Thisreduces the weight and lessens the chance of breakage.

Figure 4-9. Porro prism system.

(2) A second type of Porro system is known asthe Abbe system (fig. 4-10). It consists of a singlesystem which is three prisms in one or it can be made inthe form of two prisms (fig 4-21). It is substantially thesame in principle as the first Porro type but thereflections occur in different order. It likewise producesan inverted, reverted image.

Figure 4-10. Abbe system.

d. Amici or Roof-Angle Prism.(1) This is a one-piece prism which may be

considered as made up of a right-angle reflecting prism(fig. 2-43) with the hypotenuse face or base replaced bytwo faces inclined toward each other at an angle of 900(fig 4-11). The latter two faces form the "roof" and givethis prism its name. When inserted in the optical systemof a telescope, it bends the light rays within theinstrument through an angle of 900 while, at the sametime, inverting and reverting the image.

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Figure 4-11. Amici or roof-angle prism.

(2) When employed as the erecting prism of anelbow telescope (fig 8-11), one of its uses, the light rayenters One short face (fig 4-11) where it strikes thereflecting surface of one side of the roof. It is reflected tothe other side of the roof where it is reflected outwardlyat right angles to the direction in which it entered theprism.

Usually, the parts of the prism not used by light rays areremoved affecting the appearance of the prism but notits fundamental shape or function.

(3) The Amici prism is compact and it transmitsa great amount of light but is one of the most difficult ofall prisms to manufacture because the angle betweenthe two inclined faces of the roof cannot differ from 90°by more than 2 seconds if the prism is to functionsatisfactorily.

e. Rhomboidal Prism. The Rhomboidal orRhomboid prism (fig 4-12) may be considered as madeup of two right-angle reflecting prisms built in one piece,or as a block of glass with the upper and lower or twoother opposite faces cut at an angle of 450 and parallelto each other. This prism has two parallel reflectingsurfaces providing two reflections in the same plane andtransmits the image unchanged. It does not invert orrevert the image nor change the direction of the light raysbut displaces the light rays parallel to themselves.

This is due to double reflection without reversal of thedirection of light. This result is obtained from mirrors incertain periscopes (fig 8-19). Rhomboidal prisms areemployed to shift the lines of sight to secureinterpupillary distance adjustment. It is used in the rangefinder.

Figure 4-12. Rhomboidal prism.

f. Rotating Dove Prism. The rotating Doveprism (A, fig 4-13) resembles the rhomboidal prismexcept that the angles of its ends are opposite to oneanother and this prism serves an

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TM 9-258entirely different purpose. Light rays entering the upperface of the prism are reflected once against the longestface of the prism after which they continue along theiroriginal path out of the lower face. The image is invertedonly by the single reflection. This inversion of the imageis corrected

in panoramic telescopes by a second inversion in the900 objective prism. When this prism is rotated about itslongitudinal axis, the image rotates in the same directionwith double angular speed (para 4-7d(4)).

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Figure 4-13. Rotating prisms.-

4-15

TM 9-258g. Pechan or Z Rotating Prism. The rotating

Pechan or Z prism (B, fig 4-13) consists of two prismsseparated at an angle of 45°. Light rays entering thisprism assembly are reflected five times before emerging;twice in one element and three times in the otherproviding reversion because there are five reflections inone plane. The incident rays and the emergent rays arein alinement and travel in the same directions. Two ofthe external reflecting surfaces must be silvered. Whenthis prism is rotated about its longitudinal axis, the imagerotates in the same directions with double angular speed.

h. Pentaprism or Five-Sided Prism.(1) The pentaprism (A and B, fig 4-14) reflects

light rays through an angle of 90° by two reflections fromtwo silvered surfaces at a 450 angle to one another. Itdoes not invert or revert -he image if reflection takesplace in either the

horizontal or vertical plane (the figure shows the front ofthe object and the rear of the image). This prism isuseful in range finders where the angles of the ray mustremain constant, it is necessary to reflect light raysthrough an angle of 900 in a horizontal plane, and theimage should neither be reverted nor inverted. Theangles measured by the range finder are so small that; ifa greater number of prisms were used, any deviation inthe angles brought about by slight bending of the tubeand consequent rotation of the prisms would be sufficientto impair the accuracy of the instrument. It is necessaryto silver the two reflecting surfaces of the pentaprism asthe rays strike the reflecting surfaces at angles less thanthe critical angle and total reflection otherwise would notresult. It also is used in elbow telescopes to erect theimage and produce the 90� deviation.

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Figure 4-14. Pentaprism (diagrams show same effect obtained with mirrors).

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TM 9-258(2) In the obsolete long base height finders, the

pentaprism that would be required would be so large thatit would be difficult to find blocks of glass sufficientlylarge and homogeneous for the construction of suchprisms. Two mirrors were used to replace the tworeflecting surfaces of a pentaprism (C, fig 4-14). Thesemirrors were fastened in a rigid mount at precisely thesame angles as the reflecting surfaces of the pentaprismof the same size and produced the same effect. Theeffects of temperature changes were eliminated insofaras possible to hold the reflecting surfaces in the properrelation to each other.

(3) This prism is known as a constant deviatingprism because it deviates light constantly 90� regardlessof slight deviation of incident rays from the perpendicular.

i. Triple Mirror Prism. The triple mirror prism(fig 4-15) has four triangular faces, any three of whichcan be reflecting surfaces. It may be considered asconsisting of two triangular roofs, one above and onebelow, with the crests of the roofs at right angles to oneanother. This prism has the unique property of deviation,through an angle of 180�, any ray of light entering it. Ifthe angles of this prism are accurately made, incidentlight will be returned back along a course parallel to itsoriginal path. It produces an inversion of the image.

Figure 4-15. Triple mirror prism.

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TM 9-258j. Double Right-Angle Abbe Prism. The

double right-angle Abbe prism consists of two right-angleprisms joined as shown in figure 4-16. This prismreflects light twice at right angles to the direction of theincident ray. The image is rotated 90� two times.

Figure 4-16. Double right-angle Abbe prism.

4-6. Wedges.a. The optical wedges used in fire-control

instruments are prisms with two plane surfaces at slightangles which divert the paths of light through smallangles by refraction instead of by reflection. They areused where the angle of deviation required may be amatter of fractions of seconds and it would not bepractical to produce

such slight deviations by means of reflection surfaces.Wedges are used in the measurement systems of rangefinders and to correct or adjust the alinement of paths oflight. They are of two general shapes--rectangular andround.

b. The angle at which a wedge will deviate thepath of light is dependent upon the relative slant of itstwo faces which are inclined toward each other like thesurfaces of a common house shingle. Some wedgesused in fire-control instruments appear to be plates ordisks of glass with parallel surfaces, because the anglebetween the surfaces is so slight it cannot be detectedexcept by careful examination or by actualmeasurement.

c. Because deviation of rays of light by wedgesis produced by refraction instead of reflection, a certainamount of dispersion or separation of colors results fromthe use of wedges. For this reason achromatic wedges,composed of two different kinds of glass to neutralize thedispersion, must be used where the angles throughwhich the rays are bent are relatively large or where thework performed by the wedges is of the most exactingnature.

d. All wedges cause a certain deviation of thepath of light. Instruments employing wedges aresometimes designed so that the path of light entering thewedge has a certain amount of initial deviation which istermed the constant deviation. The constant deviationmakes it possible for the wedge to neutralize thedeviation in the path of light or divert the light at a minusangle.

e. A wedge may be caused to change the pathof light in various directions by rotation of the wedge (fig4-17). The extent to which a wedge may be made todivert the path of light may be varied, likewise, bychanging the position of the wedge with relation to theother elements of the optical system (fig 4-18).

Figure 4-17. Rotation of wedge changes direction ofpath of light.

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Figure 4-18. Extent of displacement of light may bechanged by movement of wedge.

f. Another means of varying the path of light isthrough the use of pairs of wedges which are geared torotate in opposite directions. The wedges of such asystem are referred to as rotating wedges or rotatingcompensating wedges. When the thicker edges of twoof these wedges are together (A, fig 4-19), they producea deviation twice that of one wedge and in the directionof the thicker edges of the wedges. When rotated untilthe thick edge of one wedge is toward the thin edge ofthe other (B, fig 4-19), the wedges neutralize one anotherand the wedges produce no deviation. Intermediatepositions cause a deviation which increases as the thickedge of one wedge rotates farther from the thin edge ofthe other. When both wedges are rotated through 180�(C, fig 4-19), the thick edges again are together and thewedges function as in A, figure 4-19, except in theopposite direction.

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Figure 4-19. Principles of rotating measuring wedges.

4-7. Erecting Systems.a. General.

(1) The image generally is reverted andinverted by the objective.

(2) The function of the erecting system is tobring the image upright and correct reversion.

b. Types. Erecting systems are of two generaltype, lens and prism. While most lens erecting systemsare of the same general design, differing only in thetypes of lenses used and the spacing of the elements,the prism erecting systems employ elements of a widevariety of shapes causing the rays of light to be divertedin different paths in the different systems.

(1) A lens erecting system greatly increasesthe length of an instrument. It is often used where

length in the optical system is a distinct advantage as inperiscopes. This system also is employed in instrumentswith variable magnification because different degrees ofmagnification can be obtained by changing the relativepositions of the lenses.

(2) Where compactness of instrument is adesirable feature, a prism erecting system is used. Sucha system may consist of a single element or several.The great reduction of length resulting from the use of aprism system is due to the doubling back of the path oflight. The prism erecting systems in most general useemploy Porro and Abbe prisms, the Amici or roof prism,and single and double right-angle prisms.

c. Lense Erecting Systems.(1) The simplest form of lens erecting system

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places a second convex of converging lens in the properposition to pick up the inverted image framed by theobjective and forms a second erect image which ismagnified by the eyepiece (upper, fig 4-20). In most

fire-control instruments, a lens erecting system of twoand sometimes three lenses is substituted for the singleerecting lens (lower, fig 4-20). The entire lens erectingsystem functions as a single lens.

Figure 4-20. Lens erecting systems.

(2) In a two-element lens erecting system, bothlenses usually are of compound (achromatic) type. In athree-element system, the third lens is of planoconvex ordouble-convex type and serves as a collective lens. Itconverges the light rays of the image formed by theobjective so they will pass-through the erecting lenses. Itincreases the field of view and brightness of image for agiven diameter of the lenses of the erecting’ system.Inasmuch as the collective lens is sometimes placed atthe point where the image of the objective lens is formed,and a reticle must be placed at this same point, thereticle markings are simply engraved on the flat face ofthe planoconvex lens used as the collective lens.

(3) According to the placement of the erectinglenses, a lens erecting system may increase or decreasethe magnifying power of the instrument or it may notaffect magnification. Moving the erecting lenses closerto the focal point of the objective lens and farther fromthe eyepiece increases the magnification but cuts downthe field of view. When the erecting lenses are locatedat equal distances from the focal points of the objectiveand the eyepiece there is not additional magnificationdue to erecting lenses. This method of changing thedegree of magnification is employed in instruments ofvariable power.

d. Prism Erecting Systems. Prism erecting

systems function by internal reflection. With theexception of rotating prisms, if the path of light is tocontinue in an unchanged direction, a total of fourreflections is required.

(1) Porro erecting system. This system (fig 4-9) isemployed in prism binoculars and in a number oftelescopes. It tends to decrease the curvature of thefield and its use is particularly advantageous when acompact instrument having a large field of view isdesired as is the case in observation telescopes. Thissystem is not well suited to telescopes which are to bemounted gun carriages as the prisms are of awkwardshape and it is difficult to clamp them so tightly that theywill not shift their positions when the gun is fired. Insome periscopes and in the battery commander’speriscope, the 900 prisms in the heads together with thelower prisms constitute a Ponrro prism erecting system.

(2) Abbe erecting system. This erectingsystem (fig 4-21) employs two double right-angle prisms(fig 4-16). The two prisms are assembled in theinstrument to give the same optical effect as in the threepiece Abbe prism (fig 4-10). The two prisms are notjoined, a slight amount of space being left between them.This system, which inverts and reverts, is used in sometelescopes, including those incorporated in periscopes,as an erecting system.

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Figure 4-21. Erecting system using two double right-angle Abbe prisms.

(3) Amici or roof-angle prism. When used inan elbow telescope, this single prism (fig 4-11) serves asan erecting system as well as a right angle prism. By itsuse the light rays are diverted through the right angle ofthe telescope and the erection of the image is

secured by reflection on two of its surfaces. It can bemounted securely and thus is suitable for telescopes aremounted on gun carriages. Combined with a rotatingprism and a right-angle prism it is used in the erectingsystem of panoramic telescopes (fig 4-22).

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Figure 4-22. Optical system of panoramic telescope.

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(4) Dove rotating prism.(a) The Dove rotating prism (A, fig 4-13) is the

means by which the image in some panoramictelescopes is caused to remain erect as the head of theinstrument is rotated. It usually is placed either outsidethe telescope system proper or between symmetricalerectors. The line of sight of the rotating head can berevolved in azimuth 360� (a complete circle) while theeyepiece is fixed so that the observer does not move.Some panoramic telescopes (fig 8-16) employ theobjective 90� prism in the head, and the Dove (rotating)prism is caused to revolve in the same direction andthrough one-half the arc of the 90� prism in the head.The field, inverted by the objective 90� prism, and againinverted by the Dove prism, also is rotated through twicethe arc of travel of the Dove prism itself as illustrated infigure 4-23.

Figure 4-23. Image rotation in Dove prism (end view).

(b) An object such as a vertical aiming stakewill appear upside down when viewed through a Doveprism positioned with its long face horizontal. Whenlooking forward through the panoramic telescope thisinversion corrects that initially introduced by the 90�prism in the head. As the panoramic head prism isrevolved from this forward looking position, it introducesangular tilt equal to its angular rotation so that when it isturned to look to the user’s left (for example) a verticalaiming stake will become horizontal in the image. Sincethe rotation, or tilt, of the field image produced by rotationof the Dove prism is twice the rotation of the plane of thereflecting surface (in accord with the law of reflection),this 90� tilt will be eliminated by a 45� rotation of theDove prism.

(c) As an example, consider a horizontal

aiming stake viewed through a horizontal Dove prismpositioned with the reflecting face at 45� angle to thehorizontal (fig 4-23). Since the image is reverted in theplane of reflection, it can be considered to revolve aboutthe axis AB, which is parallel to the reflecting surface. Inthis case, angle "a" is 45�; therefore, angle "b" is 45° andtheir sum is 90� or twice the angular rotation of the prismitself. When the 90� prism (in the panoramic head) isturned 180�, it will transmit an upright but horizontally-reverted image to the Dove prism. Because the field isbehind the observer, the effect is the same as in viewingthe user in a mirror where the user's image appearsreverted horizontally. In order to provide a normalimage, the Dove prism is so positioned by the 90�rotation of the plane of reflection that horizontal reversionoccurs.

(d) As both incident and emergent rays arerefracted by a Dove prism, it must be fed by parallel light,otherwise an aberration is introduced. For this reason itis usually placed outside (in front of) the telescopesystem proper. If it is desired to place a rotating prismwithin a telescope system where rays are not parallel, thePechan prism is used as its incident and emergent facesare perpendicular to its surfaces and it is, therefore, not arefracting prism and will not cause aberrations. If it isdesired to introduce a Dove prism within a telescopicsystem, it must be placed between erectors sopositioned that principal rays between erectors areparallel.

(5) Pechan or Z rotating prism. The Pechan orZ rotating prism (B, fig 4-13) can be used, similar to aDove rotating prism, as a rotating prism in panoramictelescopes to maintain an erect field by neutralizing thereverting effect of the objective prism and when therotating head is turned ( (4) above). When this prism isrotated about its axis, the image rotates in the samedirections with double angular speed. By rotating theprism, erection of the image is accomplished.4-8. Mirrors. The mirrors used to reflect the paths oflight in fire-control instruments are not the commonhousehold type made of plate glass. The reflectingsurfaces on household mirrors produce a faint reflectionfrom the front glass surface in addition to that producedby the silvered back. To eliminate this "ghost image," thereflecting surfaces of optical mirrors are placed on theirfront faces. Mirrors of this type are termed front surfacemirrors. The reflecting surface is a thin film of metalchosen for its resistance to tarnish. Such mirrors mustbe handled carefully as they can easily be scratched andbecome useless until they can be resilvered orrealuminized.

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4-9. Reticles.a. The majority of reticles (para 5-19) are glass

disks with plane parallel surfaces. Appropriate markingsare engraved or etched on one of the surfaces. In somecases, a planoconvex lens is required at the point wherethe reticle would be mounted. In such cases, themarkings are engraved on the plane surface of the lens.

b. Military reticles are of several types: wire (orsome filament material), a post (picket), an etching on aplate glass or lens surface, or a punched metal

plate as in a reflex system.(1) Crosswires (A, fig 4-24), commonly used in

rifle scopes, give increased light transmission byeliminating one piece of glass. With a wire reticle, thereis no glass surface present to become dirty and requirecleaning. Crosswires may be cleaned with little effort byusing a camels-hair brush dipped in carbon tetrachloride.A dirty glass surface in the image or reticle plane is insharp focus and under magnification. Such a glassreticle surface, therefore, must be clean.

Figure 4-24. Representative types of reticles

(2) Reticles on glass commonly are used inmost military sights. The reticle pattern is etched andfilled on a planoparallel plate so that it is seen as a blacksilhouette. If illuminated, it will glow with reflected light.

Glass provides a surface for any desired reticle design orpattern as illustrated in B, C, and D, figure 4-24. Thisglass surface must be perfectly clean as it is in a focalplace and all dirt is in perfect focus and magnified. A

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lensatic retical (reticle-lens) is one etched on the planesurface of a collective or field lens.

c. There are many different types of reticle patterns(fig 4-25) designed for different purposes. The reticle ofan instrument designed for use on a particular weapon,when used with specific ammunition, is

identified by a group of small figures and letters whichappear near the bottom or top of the field of view (C andE, fig 4-25). These figures and letters may be firing tablenumbers or F.T. numbers to indicate the firing table usedin designing the reticle or may be other characteristicidentification information.

Figure 4-25. Representative types of reticle patterns.

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4-10. Diaphragms or Stops.a. General. Diaphragms are rings of opaque

material placed in an optical system so that light passesthrough their centers. As their apertures limit the field ofview they are sometimes called field stops. Whenplaced around the edges of lenses, they prevent raysfrom passing through the margins of the lenses andcausing aberrations. When placed between theobjective and the erecting system or between theerecting system and the eyepiece, or in both or thesepositions or between parts of the erecting system,they eliminate marginal rays which would otherwisecause glare and haze by reflecting from the inside walls,

or ghost images caused by internal reflections fromcurved lens surfaces.

b. Antiglare Stops.(1) Antiglare stops improve contrast by

preventing rays exterior to the field from bouncing off theinterior of the instrument and fogging the field or causingglare ray (2, fig 4-26). Nonreflecting paint and bafflefinish on inner wall of the tube likewise eliminate most ofthis undesired light. Light from a billiant source may stillbe reflected off a dark wall and cause serious troubleespecially with a glass reticle. This is particularly true inwide-angle telescopes and antiaircraft instruments whichmay be pointed toward the sun.

Figure 4-26. Diaphragm (stop) location.

(2) In straight-tube telescopes, the stops aremerely washers or disks with a hole in the center. Theprism shelf in a binocular is designed to act as a stop.The groove in the face, the rounded corners, andflattened apex of Porro prisms are designed to functionas stops.

(3) Antiglare stops usually are located betweenthe objective and the erectors or wherever an image ofthe aperture stop is formed (fig 4-26). They are knownas erector stops if they are located between erectors.Such stops provide balance illumination by limiting therays from the center of the field to the same area asthose from the edge of the field.

c. Field Stops. The field stop (fig 4-26) limits thefield to that area which is fully illuminated and sharplyfocused by eliminating the peripheral region of poorimagery (caused by aberrations) and also prevents theobserver from viewing the inside of the instrument. It islocated in an image plane providing a sharply definedlimit to the field. It is designed to admit all the light orlimiting rays needed by the next element whethererectors or eyepiece. The field stop also may function asan antiglare stop preventing rays exterior to the field frombeing reflected off inner surfaces back into the field (ray1, fig 4-26). If a field stop is used in each image plane,the second such stop is slightly larger than the

image of the first at that point so that slight inaccuracy insize or positioning will not conflict with the sharplydefined image of the first.d. Aperture Stops. In telescopes, this stop (fig 4-26)usually is the objective lens cell or retaining ring as thereis no reason for reducing the size of the aperture of thesingle compound objective lens usually used in such aninstrument (as contrasted with the need for such stops inwide-angle camera lenses to reduce aberrations). Astop in close proximity to such a single compound lensobjective would only reduce the illumination and exit pupilsize (the same as using a smaller lens) without reductionin aberrations from the lens area used. In a complexobjective (two or more separate lenses), however, a stoplocated between the lenses may reduce aberrations.Also, in a complex eyepiece of several elements, a stopbetween the elements may serve to reduce aberrations.

e. Field Effects. All stops, excepting an objectiveaperture stop, can be considered to limit the field sincethey may prevent rays exterior to the field from reachingthe eye. In designing a system, however, this field limitis determined from the apparent field of the eyepiece sothat in the final analysis the true field of the instrumentmust be governed by the eyepiece.

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4-11. Filters.a. General. Filters or ray filters are colored glass

disks with plane parallel surfaces. They are placed in thepath of light through the optical system of a fire-controlinstrument to reduce glare and light

intensities. They are provided as separate elementswhich may be attached and detached (A, fig 4-27) or aremounted so they may be placed in or out of position asdesired (B and C, fig 4-27).

Figure 4-27. Types of filter mountings.

b. Colored Filters. Filters of various colors areused to improve visibility under different atmospheric andlight conditions. Among the colors of filters used are:smoke, yellow, amber, blue, red and greenish-yellow.

(1) The smoke (neutral) filter reduces theintensity of light and is effective when observing againstor in close vicinity of sun or a searchlight; usually it is toodark for other purposes.

(2) The yellow and amber are used to protectthe eyes from the reflection of sunlight on water andother general conditions of glare.

(3) The amber and red filters are usuallyemployed under various conditions of fog and groundhaze. Red filters are also used in observing tracer fire.

(4) The blue filter is helpful in detecting theoutlines of camouflaged objects.

(5) The greenish-yellow filter is intended toserve the purpose of both smoke and amber.

c. Polarizing filters.(1) Polarizing filters do not change the color of

objects but merely decrease light intensity and are usedto eliminate glare. When mounted in pairs, they can beused to provide continuous control of light intensity.

(2) Light polarizing substances can beconsidered as being made up of very minute parallelbars or grain (A and C, fig 4-28). Light vibrates in waveswith the crests of the waves in all directions. A polarizingsubstance, placed in the path of the light, permits wavesvibrating parallel to the direction of the grain to passthrough; waves vibrating at right angles to the bars orgrain are stopped (B, fig 4-28). By eliminating a portionof the light, the intensity of the light is decreased. Whentwo polarizing substances are held with the grain of oneat right angles to that of the other, no light passesthrough. By shifting the substance to intermediateangles, light at different degrees of intensity is permittedto pass through.

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Figure 4-28. Principles of polarization of light.

(3) Glare is due to light which is reflected fromsmooth or wet horizontal surfaces. It can be veryannoying and detrimental to clear vision. Such light asbeing reflected at a glancing angle from a smoothhorizontal surface is found to be also polarized in thehorizontal plane, the vertical waves being substantiallyeliminated. A polarizing filter consisting of vertical grainor grid will destroy the remaining horizontal component ofthat portion of the light which causes glare whilepermitting normal vision to continue practicallyunobstructed. The principal function of polarizing filtersis to eliminate glare in the field of vision.4-12. Lighting.

a. The reticles of fire-control instruments requireillumination for night operation and under certainconditions for day operation in order that their markingsmay be seen clearly.

b. Reticles are edge-lighted. The markings areetched or engraved into the surface of the glass. Whenlight is introduced at the edge of the reticle, it travelsthrough the glass and is diffused at these etched orengraved marks illuminating them. Light is introduced atthe edges of the reticles through small glass windows setin openings in the body of the instrument. At night, tinyshielded electric lamps are placed over the windows.

c. Lucite is a transparent plastic having theproperty of transmitting light which enters one end of arod of this material. The light travels from end to end ofthe rod with little loss through the outside surface,despite the fact that the rod may be bent. Lucite isemployed in the lighting systems of a number ofinstruments (fig 4-29).

Figure 4-29. Instrument light for panoramic telescope.

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d. Instrument lights for the majority of fire-controlinstruments are detachable. They consist of a containerfor one or more dry-cell batteries, a light switch which issometimes combined with a rheostat, a clampingarrangement, and a miniature electric bulb with facilitiesfor directing the illumination to one or more points on theinstrument (fig 4-29). Some systems have an additional"finger light" (fig 4-30) to permit directing light to any partof the instrument.

Figure 4-30. Instrument light for BC periscope.

e. Some instrument lights are not supplied with dry-cell batteries, but use the vehicle power source.

Section II. COATED OPTICS

4-13. Introduction.a. Light loss by absorption and scattered dispersive

reflection on a silvered surface is about 7 percent.Whenever light strikes an ordinary glass surface, most ofthe light is transmitted through the glass and undergoesrefraction. However, even under the most favorableconditions generally about 4 to 6 percent of the light islost by reflection from the surface. This loss takes placewhen light passes from glass to air as well as when lightpasses from air to glass.

b. This condition occurs at each optical surface ofthe elements in a fire-control instrument. For example,in a straight telescope with 6 optical elements there are12 surfaces at which there is reflection and loss of light.Consequently, if each surface transmits 96 percent of thelight which it receives, only about 60 percent of the

light which enters the telescope is transmitted to the eye;the remaining 40 percent is lost by reflection andabsorption at the surfaces of the various elements. Theresult is that the image becomes dimmer, the strayreflections produce glare which reduces contrast, and"ghost images" are produced when viewing bright objectsat night due to reflection from the curved lens surfaces.

c. A process has been developed for coating theair-to-glass surfaces of optical elements with a very thinfilm of transparent substance. The coating reduces theloss of light by reflection from each optical interfacesurface to less than 1 percent (about 1/2 percent),resulting in a corresponding increase in the amount of4ight transmitted through the element. Optical elementsso treated are termed coated optics.

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d. A number of types of coating have been testedand applied to optical elements. Some have proved toosoft to stand up under handling in normal use andservice. The present transparent coating consists ofmagnesium fluoride applied by an evaporation method ofsublimation under heat in vacuum. The coating isdurable enough to permit handling during assembly anddisassembly and to withstand careful cleaning.

e. The main advantages of coating are as follows: (1) Increased light transmission. When all elements

of a straight telescope are coated, except the reticle andfilters, there is approximately 90 percent transmission oflight instead of 60 percent, or half again as much light aswould pass through the instrument were its opticsuncoated. This improvement will result in a brighterimage at times when light is scarce. It will be mostobvious at dawn, dusk, or at night; in other words, attimes of poor visibility. Targets are visible from 1/4 to 1/2hour longer at dusk and at dawn. The range of vision atnight is increased approximately 20 percent for standardbinoculars. Therefore, coating is essential in nightglasses or in instruments to be used under adverseconditions and now is used in all military optical systems.

(2) Reduction of haze. Light reflected from

the surfaces of optical components in an instrumenttends to throw a veil of stray light over the field of view.Coating will reduce these internal reflections. Bettercontrast and sharper definition of the image will be theresult even under intense illumination.

(3) Reduction of ghost images. Internalreflections from the curved lens surfaces sometimesform an image or series of images which are slightly outof focus and not quite as bright as the image of focus.These ghost images are very distractive for properaiming. Coating will greatly reduce the formation ofghost images.

(4) Reduction of front surface reflections.Reflections from the front surface of observationinstruments are very noticeable at night and might givethe observer’s position away. By reducing reflections bymeans of coating, this danger is reduced.4-14. Theory.

a. Consider a tiny portion of a light ray so highlymagnified as to show its wave structure. A ray of lightstriking an uncoated glass surface could then be picturedas a wavy line as shown in figure 4-31. The reflectedray, which can represent 4 percent of the total light, isrepresented by the wavy dotted line and will shift inphase 180° (reverse direction of vibration).

Figure 4-31. Light ray striking uncoated surface.

b. If a thin film of transparent substance wereplaced on the glass, there would be two surfaces fromwhich the wave would be reflected and similarly shifted inphase 180�. If this film were one-half as thick as awavelength of light, the reflected waves from the twosurfaces would follow the same path (fig 4-32). In doing

so, they would reinforce each other resulting in thedouble amount of light being reflected and the minimumamount of light being transmitted through the surfaces.This condition would be desirable in a reflector butundesirable in a lens.

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Figure 4-32. Light ray striking coated surface (coating one-half wave length in thickness).

c. If, however, the film were only one-quarter of awavelength in thickness, the reflected waves wouldfollow paths which would cause them to interfere witheach other and the reflected waves would cancel

each other (fig 4-33). The radiant energy, which is light,is not lost when the reflected waves are canceled;instead, this energy is contributed to the refracted light.

Figure 4-33. Light ray striking coated surface (coating one-quarter wavelength in thickness).

d. Another way to understand the action of coatedoptics is to consider two incident light waves: ray 1 andnext oncoming wave, ray 2. With a film of one-quarterwavelength in thickness, the time required for a wave inray 1 (fig 4-34) to travel from the top of the film to thebottom and return results in a one-half wavelength lagrelative to the reflected wave from the next oncomingwave, or to the coincident reflected wave in ray 2 at the

top surface so that the two are 180� out of phase. Thevibrations of these two coincident reflected waves thenare in opposite directions and cancel each other, thesame as two matched teams in a tug of war, pulling withequal force, remain stationary. Because of the 180°

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phase shift previously mentioned however, the oncomingwave in ray 2 coincident (at the air-to-film interface) withthe wave from ray 1 (reflected from film-to-glassinterface) i will be in phase. Reinforced transmission ofray 2 will result.

Figure 4-34. Low-reflectance coating.

e. Since the wavelength is different for eachdifferent color of light, one color must be selected todetermine how thick the coating shall be. Green light,with a wavelength of 20 millionths of an inch, is thelogical color to use since it is in the middle of the visiblespectrum. This color is in the brightest region andcontributes most to vision. The thickness of the coating,therefore, should be 1/4 of 20 millionths of an inch, orequal to 5 millionths of an inch and result in increasedtransmission of green light and adjacent colors(diminishing toward outer colors in spectrum). Reflectionof the colors at the extreme ends of the spectrum (redand violet) is not eliminated completely. Therefore, lightreflected from a coated optical element appears purplish,similar to a mixture of red and violet regardless ofcoating material used. Thus presence of coating isapparent to visual inspection. The common soap bubbleor oil slick on water reflects iridescent colors because ofthe varying thickness of the film.

f. Another useful application of magnesium fluoridecoatings is on front surface aluminized mirrors used inoptical instruments. In this case, a film thickness of one-half wavelength causes reflected waves to reinforceeach other (fig 4-32).Such coatings also may be applied to front surfacesilvered mirrors. An aluminum or silver surface coated inthis manner is no longer soft and easily scratched, butbecomes harder and more durable and may be cleanedlike other optical elements.4-15. Identification

a. Whether or not an instrument is fitted withcoated optics can be detected readily by holding theinstrument at an angle to a source of light and observing

the reflections from the eyepiece and objective endsurfaces. If the optical elements in the instruments arecoated, the reflection of light will have a distinctivepurplish tinge. Under certain conditions, the faces oflenses may appear to have a dull film or patina.

b. An alternative method is to compare theillumination of the field of view of an instrument with thesame model number of series known to have’ opticalelements that are uncoated. The instrument with coatedoptics will have a much brighter field.

c. All fire-control instruments with coated opticsbear a decalcomania transfer with the following wording:"This Instrument Has ’COATED OPTICS’. Clean LensesCarefully."

d. The magnesium fluoride coating is referred to ashard coating. There are a few instruments in the fieldwith elements coated with what is referred to as softcoating. The coating on these elements is not asdurable as the hard coating. A simple test will distinguisha hard coating from a soft coating. With a soft rubbereraser, rub the coated element near the edge so as notto impair the usefulness of the element. About 20strokes will remove a soft coating, but hard coating willnot be affected.4-16. Serviceability.

a. Hard coating films on coated optics are toughand durable. They are insoluble in water, are notaffected by oil and alcohol, and salt water will not harmthem if cleaned off promptly. The coat will withstandtemperature from -60 to +200� F. They are specified towithstand a rubbing of a 3/8-inch pad of dry cotton,exerting a force of 1 pound rubbed in any direction 50times. However, due to the critical significance of thethickness of the coating the effectiveness of the coatingmay be completely destroyed by carelessness,ignorance, or rough treatment.

b. Scratches tend to lower the quality of the lens.Wearing down of the film due to repeated cleaning is notharmful to the element itself, but a reduction in thicknessof the film much below one quarter of a wavelength ofgreen light reduces its efficiency as a coated opticalelement. Partial or complete removal of the coating doesnot make the optical element useless but merely partlywholly takes away the benefit of the coating, leaving theelement as if it had not been coated in the first place. Itis important that remnants of a partly deterioratedcoating should not be removed, since even a partiallycoated lens will be more effective than an uncoated one.

c. Coated optics slowly deteriorate under finger-prints, under prolonged action of atmospheric dust andmoisture, and salt water. Instruments containing coatedoptics should be well sealed. Exceptional precautionsmust be taken to prevent sealing and cementing com-

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pounds from getting on the coated surfaces of lenses.The removal of such compounds undoubtedly wouldresult in injury to the coated surfaces.

d. Every precaution should be taken to preventinexperienced personnel from misunderstanding thefunction of the coating and attempting to remove it.

Section III. GENERAL CONSTRUCTION FEATURES

4-17. Lens Cells, Separators, Lens RetainingRings, and Adapters.

a. A lens cell (fig 4-35) is a tubular mounting orframe made of metal, plactic, or hard rubber which holdsa lens or a number of lenses in the proper position withinan optical instrument. The lens usually is secured intothe cell by a lens retaining ring or by turning over a

thin edge of the cell (burnishing). A setscrew locks theretaining ring in position in the lens cell. When two ormore lenses are mounted in a cell, the different elementsare maintaintained at the proper distance from oneanother by spacers of separators. The cell mountingpermits the entire assembly to be handled and mountedas a unit.

Figure 4-35. Lens cell, separators, lenses, and retaining ring.

b. Separators or spacers are smooth or threadedtubular sections which separate or space the elements ofa lens system in the proper relation with one another (fig4-35). Adjoining faces are usually beveled to fit the facesof lenses snugly.

c. Lens retaining rings are generally threadedabout their outer diameters and are screwed in overlenses to secure the lenses to their cells.

d. Adapters may be used to mount an element orpart of smaller diameter into a part of the instrumentbody which is of larger diameter.4-18. Centering Devices.

a. Light always bends toward the thickest part of a

lens or prism. When the center of a lens is moved it canbe made to cause the light to be bent in a differentdirection. In cases where extreme exactness is required,proper adjustment of the center of the objective isaccomplishing by mounting it in a pair of eccentric rings(fig 4-36). By rotating the inner ring about in the outerring, or by rotating the outer ring in the lens cell, the axisof the objective may be moved to any point in a relativelylarge area. When properly positioned, the rings may belocked in position. Skill is required to set the rings in theproper manner.

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Figure 4-36. Eccentric objective mounting.

b. Prisms were formerly often placed in mountingsdesigned to hold them firmly and yet afford movement forexact adjustment. Screws bear upon two faces of theprism to lock it in place in such mountings. By letting outon one screw and taking up on the other, the prism maybe positioned exactly. Extremely fine adjustments canbe made. Care must be exercised in tightening screwsto insure that prisms will not chip or crack from excessivepressure.

c. In modern instruments, mirrors and prisms arebonded to metal brackets or holders. Adjustment of thevarious components is accomplished by the screwsbearing against the metal holder.4-19. Eyeshields. Eyeshields or eyeguards are fittedto fire-control instruments to maintain proper eyedistance and to protect the eyes of the observer fromstray light, wind, and injury due to the shock of gun fire orsimilar disturbances. Eyeshields are made of rubber,plastic, and metal but only those made of soft rubber canmost effectively meet all of these requirements.4-20. Sunshades and Objective Caps.

a. Sunshades are tubular sections of metal, usuallywith the lower portion cut away, which are fitted into slotsaround the objective cells of many instruments to protectagainst rain and the direct rays of the sun (fig 4-37).They not only reduce glare which would be

caused by sunlight striking the outer face of theobjective, but they protect from extreme heat thethermosetting cement used to cement the elements ofthe objective.

Figure 4-37. Sunshade and objective cap.

b. Objective caps (fig 4-38) are leather coverswhich are fitted over the sunshade or objective end ofthe instrument to protect the objective when theinstrument is not in use. The caps are usually attachedto the instrument by strips of leather to prevent loss.4-21. Focusing Devices. The distance between thereticle and the eyepiece must be adjusted to theobserver’s eye so that the reticle and image of the objectwill be sharply defined and to eliminate eye fatigue. Toprovide this adjustment, the two or more lenses of theeyepiece are mounted in a single tube or lens cell and itsdistance from the reticle (and the focal plane of theobjective) can be adjusted by a rack and pinion, by asimple draw tube, or by rotating the entire eyepiece whenadjusting the diopter scale (fig 4-38) causing it to screwin or out. This is referred to as the diopter movement(para 5-14). The knurled ring of the "screw out"adjustment type is provided with a scale reading indiopters by which this adjustment can be made directly ifthe correction required by the eye is known.

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Figure 4-38. Diopter scale.

4-22. Optical Bars.a. In a range finder, the angles of the lines of sight

from both ends of the instrument are employed todetermine the range. Heat and cold cause unequalexpansion and contraction of metal parts disturbing theangular alinement of elements mounted on or in them.The angles determined by the range finders are so smallthat an excessive error could be introduced by thedeviation caused by any misalignment if specialmeasures were not taken to keep the optical systemsstable and in perfect alinement.

b. One of these measures consist of mounting themost sensitive parts of the instrument in a metal tubeknown as the optical bar. The optical bar is made ofspecially composed metal with a low coefficient ofexpansion. It is perfectly balanced and is supported bythe inner or main tube of the instrument in such amanner that the outer tube may expand or contractwithout affecting the alinement of the inner tube. Inaddition, the instrument is insulated to reducetemperature changes to a minimum.

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CHAPTER 5

ELEMENTARY PRINCIPLES OF TELESCOPES

Section I. INTRODUCTION

5-1. History of Optical Instruments. The earliestrecord of the use of a lens as a magnifier is estimated tohave been in the eleventh century and attributed to anArabian philosopher names Alhazen. In the thirteenthcentury, spectacles were devised based on work doneboth by Roger Bacon and by a Florentine monk named diSpina. But as the seventeenth century opened, nobodyhad as yet devised a telescope. Quite by accident aDutch optician, Jan Lippershey, made the discovery in1608. His telescope used a 1 1/2-inch diameter positive(convergent) objective lens of 18-inch focal length and anegative (divergent) eyelens of 2-inch focal length.Lippershey was refused a patent by the Dutchgovernment because of confusing counterclaims fromsupposed competitors but in the spring of 1609 theItalian scientist Galileo carried on the work. Heduplicated the original using an instrument of 8 power.Not satisfied, he made another instrument 37 inches longof 8 power and still another 49 inches long with a veryshort focus eyepiece giving 33 power. His famousastronomical discoveries were made with the latterinstrument and the optical system he used is still knownas the Galilean telescope. A refracting astronomicaltelescope of modern design, using a positive objectivelens, and a positive eyelens, was designed by theGerman astronomer Kepler in 1610 and some 30 yearslater a Scottish mathematician named Gregory designedthe first reflecting telescope which is the basis of manymodern astronomical instruments. In the latter part ofthe nineteenth century, buffalo hunters on our westernplains used telescopic sights on their rifles. Thetelescopic sight, the most accurate of all sightingdevices, is too easily damaged to be an item of issuewith the infantry rifle except those weapons used bysnipers. It is used, however, on all types of field piecesand Naval rifles. The remainder of this text is devotedmainly to a study of the principles involved in suchapplications of telescope as telescopic sights andobservation instruments.5-2. Simple Magnifier. The simple magnifier consistsof a positive (converging) lens at the first focal plane ofthe eye, although this positioning is not too important. Ifthe object being viewed is at or within the focal point ofthe lens, a virtual, erect, and enlarged image is seen by

the eye. It should be remembered that rays from adistant point are, for all practical purposes, parallel andthat the eye can focus or form an image with theseparallel rays without accommodations. If the object is atthe focus (focal point) of the lens, it is seen by the eyewithout accommodation as the emergent rays are thenparallel (as if originating at a distant object) as illustratedin figure 5-1. If the object is within the focal length of thelens, accommodation by the eye is necessary as theemergent rays no longer are parallel but are diverging,as if from a near object, as in figure 5-2.

Figure 5-1. Simple magnified-object at principal focus

Figure 5-2. Simple magnifier-object within focal length

5-3. Compound Microscope. The real purpose of themicroscope is to aid the human eye, essentially a long-range high-acuity instrument, to be useful at short dis-

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tances. If it is recalled that apparent size equals actualsize divided by distance to object, it should be apparentthat the closer an object is to the eye the larger will be itsimage. The microscope uses an objective lens ofextremely short focal length (fo) which forms a real,enlarged, and inverted image of an object placed justbehind its principal focus. To view this image, a positiveeyepiece of short focal length (fe) is used as a magnifier.If the real image is at the first principal focus

of the eyelens, the image is seen at infinity and noaccommodation is required. Final image may be formedat any distance from the eyelens exceeding the shortestdistance of distinct vision (about 10 inches). In this case,the image formed by the objective lens must be withinthe principal focus of the eyepiece as in figure 5-3.Magnification in a microscope depends on the focallengths of the objective and the eyepiece and thedistance between them.

Figure 5-3. Simple compound microscope

5-4. Telescopes. Most persons think of telescopes asmagnifiers but they may be designed to provide the eyewith an image the same apparent size as the object orthey may provide the ’eye with a smaller image which isan example of reduction. The primary purpose of thetelescope is to improve vision of distant objects. In itssimplest form, it consists of two parts; a lens or mirror

called either the objective or objective glass (if a mirror)which usually forms a real image of the field or areawhich the telescope can "see," and an eyelens oreyepiece used to view this image. This generaldescription is exactly the same for the microscope,except that in the microscope the first image is actuallylarger than the object while in the telescope the firstimage is smaller than the object.

Section II. ASTRONOMICAL TELESCOPES

5-5. Refracting Telescopes. Refracting astronomicaltelescopes use lenses to form images through refraction.A positive lens alone will form only real images of distantobjects. Such real images in space

cannot be brought to focus by the eye (fig 5-4). The eyemust be fed either by parallel rays or rays only slightlydiverging as if from an object no closer than the nearpoint of the

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eye. However, if another positive lens is placed betweensuch an image and the eye, and the real image lies atthe first focal point of this eyelens, the eye can seewithout accommodation of a virtual image of the objectseen by the objective lens. This is the Keplerian system(fig 5-5). Such an arrangement is a telescope of thesimplest form and is illustrated in figure 5-6 with thevirtual image moved in to the near point of the eye. Thissystem generally is limited to astronomical observationbecasue the image is inverted. The Army uses thissystem only to adjust other types of telescopes and whenit is so used, calls it a collimating telescope. Figure 5-4. Objective lens.

Figure 5-5. Keplerian system

Figure 5-6. Refracting astronomical telescope

5-6. Reflecting Telescopes.a. Types of Spherical Mirrors.(1) Convex. In this case, the mirror surface is

on the outside of a spherical surface, the center ofcurvature of which lies on the side opposite the incidentlight. This type of spherical mirror produces small virtualimage only. It is frequently used on trucks as a rearviewmirror.

(2) Concave. In this type, the light is incidenton the same side as the center of curvature (C) of thesphere. The focal point (f) is located halfway betweencenter of curvature and reflecting surface (fig 5-7). Theconcave mirror forms virtual and enlarged images ofobjects within its focal length which is one-half the radiusof curvature. Some of the uses to which it is put areheadlamp reflectors, searchlight reflectors, and objectiveglasses in astronomical telescopes.

Figure 5-7. Concave mirror

(a) An object beyond center ofcurvature forms a real image between center ofcurvature and focal point. This image is smaller than theobject and inverted and reverted.

(b) When the object is between thecenter of curvature and the focal point, the image isformed beyond the center of curvature and is real,enlarged, and inverted and reverted.

b. Reflecting Telescope. This telescope,diagramed in figure 5-8, utilizes a concave mirror of longfocal length (instead of an objective lens) to form the realimage. This is viewed as a virtual image through aneyepiece which magnified it or is photographed by acamera attachment. The Mt. Palomar telescope, forexample, has a 200-inch diameter mirror of long focallength, great light-gathering ability, and great resolvingpower. (For resolving power see para 5-30).

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Figure 5-8. Reflecting astronomical telescope.

5-7. Magnification of Telescopes.a. Computation of power in an astronomical

telescope is done by dividing the focal length of theobjective by the focal length of the eyepiece. This is trueonly when the virtual image is at infinity or whenemergent rays from point object are parallel. If theimage is moved to the near point of the eye (10 inches),it increases slightly in size.

b. This equation cannot be applied to allterrestrial systems using lens erecting systems sincesuch erecting systems can, and usually do, contribute tothe power. It can be applied to any terrestrial systemusing a prism erecting system.

Section III. TERRESTRIAL TELESCOPES

5-8. General. This instrument gets its name from theLatin term "terra" which means earth and is basicallyuseful for looking at objects as they actually appear to uson earth. Any astronomical telescope can be convertedinto a terrestrial telescope by inserting an erectingsystem. either lens or prism, between the objective and

the eyepiece to erect the image as in figure 5-9. A prismerecting system is placed between the objective and itsfocal point, but a lens erecting system requiresrepositioning of objective and eyepiece so that erectorsare between objective focal point and first principal focusof eyepiece.

Figure 5-9. Terrestrial telescope—simple form.

5-9. Lens Erecting Systems.a. Location. A lens erecting system is placed

between the focal plane of the objective and the frontfocal plane of the eyepiece. In practice, to minimizeaberrations, two or more lenses usually comprises a lens

erecting system. The first lens may be placed one focallength behind the image formed by the objective and thesecond spaced a distance equal to the focal length ofeither to minimize spherical aberrations (fig 5-10).

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Figure 5-10. Symmetrical erectors.

b. Symmetrical. A true symmetrical erectingsystem is composed of symmetrical lenses so positionedthat the object (real image formed by the objective) andimage distances are symmetrical or equal (fig 5-10). Inother words, the image formed by the objective is at thefocal point of the front erector and the rays between thetwo erectors are parallel. Therefore, spacing betweenerectors is not critical and does not affect magnificationas the magnification in such a system (always-1)indicates an inverted image of same size as objectregardless of lens separation.

c. Variable Magnification. Variablemagnification erecting systems used in variable powertelescopes may provide (in the high-power position) fromtwo to three times the power produced in the low-powerposition depending on the design. In the simplest lenserecting system (a single lens), magnification can beshown to be a function of object distance and imagedistance (A and B, fig 5-11, and para 2-24e). The sameis true if a combination of lenses (usually two) is used.

Figure 5-11. Simple erector.

(1) Magnification of an erecting systemcomposed of a combination of asymmetrical lenses(lenses of different focal lengths) can be varied either byvarying the distance of the erecting system from itsobject (image formed by objective) or by varying the

separation between the elements in the erecting system(A, B, and C fig 5-12).

Figure 5-12. Asymmetrical erectors.

(2) When magnification is increased byshifting erectors forward, the image position shifts towardthe eyepiece as in B, fig 5-12. A change in distancethen, between the asymmetrical elements in this forwardposition, further affects magnification and imagedistance. A decrease in the distance between theelements, i.e., moving the second lens nearer the first,lessens the increase in magnification slightly but moreimportant decreases the shift in the resulting imageposition as in C, figure 5-12. By design, the imageposition can be the same for any of the possiblemagnifications. In this case, the separation betweenerector elements is always such that the image positionremains fixed for any magnification and the eyepiece canbe fixed (C, fig 5-12).

(3) If magnification is increased, either byshifting the erector system forward as a complete

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unit or by moving only the rear erector element forward,the image formed by the erectors will move back. In thiscase, it is necessary to move the eyepiece back adistance equal to the image shift to keep the image infocus (B, fig 5-12).5-10. Prism Erecting Systems. A prism erectingsystem (para 4-7d) may be used in a terrestrial telescopeinstead of a lens erecting system. In prism, offset, andelbow telescopes, it is placed between the objective andits focal plane. Either all or part of the prism erectingsystem, however, may be outside (in front of) thetelescope system. In the BC periscope, the prism in thehead is outside the periscope system but still a part ofthe prism erecting system. The prism in the head,together with the prism system in the elbow below,comprises a Porro prism erecting system. All observingand sighting telescopes used by the Army employ anerecting system.5-11. Galilean Telescope.

a. Basic Principles. The system employed inthe Galilean telescope is based on two major principles.

(1) It makes use of a negative eyelenspositioned a distance equal to its focal length (fe) in frontof the objective focal point (figs 5-13 and 5-14).

This negative eyelens is placed so that rays from theobjective meet it before forming the real image, whichdoes not exist, and is called the virtual object. Raysapproaching this virtual object will be bent outward andmade to diverge as if from the enlarged virtual image (fig5-13). This renders converging rays from the objectiveparallel before they have converged to form a realimage; therefore, no real image exists in this system.The real image, which would be formed by the objectiveif the eyelens weren’t there, becomes the virtual objectfor the eyepiece and lies at the first principal focus of theeyelens. In this case, the virtual image viewed throughthe negative eyelens will be at infinity and can be viewedby the eye without accommodation. This is the zerodiopter setting (fig 5-14) for this instrument, meaning thatall rays from any point source emerge from the eyepieceparallel. If the eyelens is moved in or out, the emergentrays either will converge or diverge so that the instrumentcap be adjusted for farsighted or nearsighted eyes or fordistance. The image viewed through the eyepiece iserect because the emergent rays are bent further awayfrom the axis instead of recrossing it as in a Keplerian orastronomical system employing a positive eyelens.Compare figure 5-13 with figure 5-6.

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Figure 5-13. Galilean system.

Figure 5-14. Relations of elements in Galilean telescope(zero diopter setting)

(2) This is the only telescopic system inwhich the diameter of the objective controls the field ofview (width of visible observed area), as the objective isboth field stop and entrance window (fig 5-15). Thissystem is, therefore, limited to small fields and low powerand usually is designed for 2 or 3 power.

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Figure 5-15. Objective diameter limits field in Galilean system

b. Uses. This system is quite inexpensivebecause of its simplicity and is used in field glasses. It isused in opera glasses because of its very short lengthand light weight. Note that is provides an erect imagewithout lens or prismatic erectors in a system shorterthan a modern astronomical system of equal power. It

is used frequently as a reduction view finder on camerasby reversing it with the positive element used as theeyelens and the negative as the objective. Because of itslow power limited field, and lack of real image, plane, ithas no application in military instruments in this country.

Section IV. FUNCTIONS OF COLLECTIVE LENSES AND EYEPIECES

5-12. Functions of Collective or Field Lenses.a. Primary Functions. This element (usually

plano-convex) may be placed in a system with the planoside coincident with the image plane (focal plane) of theobjective. If the principal plane of this lens lies in theimage plane of the objective, this lens does not alter thepower of the instrument but reduces the power if placedcloser to objective (fig 5-16). The collective lensprovides balanced illumination of the entire field. Rim

rays would miss the erectors without it and leave theouter portions of the field with less illumination than thecentral portion. The clear aperture required in the lensesin the erecting system is reduced so that smaller lensescan be used or the collective lens can be considered toincrease the field, if the erectors are of sufficient size andthe eyepiece has sufficient apparent field.

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Figure 5-16. Function of collective lens.

b. Secondary Functions. This lens also shortens theeye reflief (distance from eyelens to eye) of theinstrument by shifting the plane of the exit pupil (in whichthe eye must be located to view the full field) closer tothe instrument. It is not used, therefore, in rifle scopes orgunsights where long eye relief is essential to’ avoidinjury from the recoil.

5-13. Functions of Eyepiece or Ocular.a. General. An eyepiece in its simplest form

may consist of a single lens (simple magnifier) whichpresents to the eye an enlarged virtual image of the realimage formed either by the objective or the erectors asillustrated in figure 5-17. The usual apparent fieldprovided by an eyepiece is 400 to 500. A wide angleeyepiece is one fully corrected to the edges and ofsufficient aperture so that in a system of given eye reliefa wide field of up to 75. may be provided. Becausemagnification is angular, this apparent field limits the truefield of the instrument for any certain power. Forexample, a 4-power instrument with an eyepieceproviding a 400 apparent field will have a true field of100. This will be discussed fully in paragraphs 5-21through 5-31.

NOTEThe power of an eyepiece is inverselyrelated to its focal length; i.e., high-power eyepiece is one of short focallength. It is thus possible to replace aneyepiece with one of shorter focal lengthand increase the power of theinstrument.

Figure 5-17. Function of eyepiece.

b. Function of Elements. In a simple eyepieceusing two lenses, an eyelens next to the eye provides allor most of the magnification depending on the location ofthe filed lens relative to the real image. The field lens (fig5-18), if in the image plane, has no effect onmagnification but acts as a collective lens providingbalanced illumination of the field. Without the field lensprincipal rays from the edge of the field would miss theeyelens and leave the edge of the field dark. With thefield lens in the image plane, however, any dust on itssurface lying in the image plane will be in sharp focus.For this reason, unless there is an etched reticle on thissurface, the field lens usually is displaced slightly fromthe image plane thus contributing to the magnification.Another reason for not placing the field lens in the focalplane is that this is the position normally occupied by thereticle in those instruments emplying reticles with thereticle pattern etched on glass or a

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crosshair mounted on a reticle holder ring. If the eyelensand the field lens are of the same power (focal length)and the field lens lies in both the image plane and thefirst focal plane of the eyelens, the image will be viewedat infinity (without requiring accommodation by the eye).

NOTEA field lens in the image plane shortenseye relief by shifting the plane of the exitpupil without affecting its size (para 5-27b). The field lens also increases theapparent field of the eyepiece.

Figure 5-18. Function of field lens in eyepiece.

Section V. ADJUSTMENTS

5-14. Diopter Movement.a. Zero Setting (Normal). In a well-adjusted

instrument the beams of light from each point in a distantfield or target will emerge from the eyepiece (of focallength fe) as a beam of parallel rays. This is a zerodiopter setting (para 10-1), as diagramed in figure 5-19,and will feed the eye with the same parallel light as thatreceived from distant points by the unaided eye. Thenormal or emmetropic eye, therefore, can view the fieldthrough the instrument without accommodation.

Figure 5-19. Zero diopter setting.

b. Minus Setting (Shortsighted). Theshortsighted or myopic eye will form an image in front ofthe retina. Movement of the eyepiece toward the reticleand the real image under observation, will causeemergent rays from the eyepiece to diverge and correctfor the deficiency of the eye. This is a minus dioptersetting and is diagramed in figure 5-20.

Figure 5-20. Minus setting for shortsighted eye

c. Plus Setting (Farsighted ). Farsightednessor hypermetropia results in image formation in back ofthe retina. Movement of the eyepiece away from therecticle and the real image under observation, will causerays emerging from the eyepiece to coverge so that thefarsighted eye can focus the image clearly. This is aplus diopter setting and is illustrated in figure 5-21.

Figure 5-21. Plus setting for farsighted eye.

d. Low-powered Instruments. If theinstrument is less than 4x, the eyepiece can be fixedfocus for satisfactory use. This means adjusting the

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eyepiece to a single focus setting at -0.75 to -1.0 diopter.A single focus setting will be satisfactory to any observerwhose eye correction is not too great. In a low-powerinstrument, the objective is short focal length; therefore,the image plane shift is small when target distancechanges unless the target is moved very close.

e. High-Powered Instruments. In high-powerinstruments (more than 4x) the diopter movement isuseful because the eye relief frequently is too short(excepting in rifle scopes and gunsights) to permit viewof full field while wearing spectacles. If the user’sdefective sight is not caused by astigmatism (is eitherfarsighted or nearsighted) and the correction, required iswithin the range of the ’diopter movement, the diopteradjustment permits adjustment of the eyepeice to correctfor his defective vision. This eliminates the necessity forspectacles while using the instrument. In a high-powerinstrument not having a reticle, the diopter movementpermits focusing the eyepiece on the different imageplanes, resulting when the instrument is used at allranges, thus focusing the instrument for the differentranges. An example of this is the observation telescopewhich can be used from approximately 50 feet to infinity.

f. Diopter Scale. The diopter scale on aneyepiece is calibrated in units indicating the movement ofthe eyepiece necessary to effect a change of 1 diopter inthe emergent light. For example, at plus 1 the angularconvergence is the same as with a 1-power lens (focallength of 1 meter) and the rays will focus or cross at 1meter behind the eyepiece. At minus 1 the angulardivergence is such that the rays, if extended back intothe instrument, would cross at 1 meter forward of theeyepiece.5-15. Dioptometer. The dioptometer (fig 5-22) is aprecision instrument useful in determining accurately thediopter setting of another instrument. It is placedbetween the eye and the instrument to be checked. Thefocusing sleeve of the dioptometer is then adjusted untilthe field is in sharp focus. The diopter setting of theinstrument being checked is then read from the focusingsleeve (usually calibrated in 0.1 diopter) of thedioptometer. Set at zero diopter the dioptometer can beused to check-the accuracy of the zero diopter setting inthe collimating telescope itself which is commonly

available for checking zero diopter setting.

Figure 5-22. Dioptometer

5-16. Collimation.a. Definition. Collimation is the alinement of

the optical and mechanical axes of an instrument andnecessitates a reticle in the collimating telescope so itcan be used as a telescopic sight. (for reticles, see para5-19). Alinement can be obtained by sighting through afixed collimating telescope and the instrument to becollimated, and adjusting accordingly.

b. Lens Decentration. Lens decentration isutilized frequently in collimating optical systems,especially binocular instruments, which usually employobjectives mounted in eccentric rings (para 4-18) topermit movement in any direction across the axis. If alens is decentered (moved across the axis of the system)it will cause an image shift in the same direction. Theoptical center seldom coincides with the exactgeometrical center of a lens so that an image shiftfrequently will result from revolving a lens in its cell. Infact, disturbing any lens in a collimated system willnecessitate collimation of the system again as any lenswhich fits freely enough to facilitate removal will almostsurely not go back in its exact position.

c. Prismatic. If prisms are employed in asystem, collimation also can be affected either by slidingprism parallel to the plane of reflection (causing animage shift in same direction) or by tipping prism andplane of reflection (shifting image in same direction).

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Section VI. FUNCTIONS OF TELESCOPIC SIGHTS

5-17. Definition. Terrestrial telescopes having areticle in a focal plane in the optical system arecommonly known as telescopic sights or militarytelescopes. Such a reticle can be placed in anastronomical telescope, as in the collimating telescope,but a telescopic sight is universally considered to be aninstrument having an erecting system.5-18. Advantages. A telescopic sight isadvantageous in that it permits the viewing of reticle andtarget in the same optical plane and does not requireprecise alinement of the eye with respect to the line ofsight. Alinement of the eye need be only within the exitpupil diameter. In comparison, open rifle sights requirethe eye to attempt an actual impossibility; the focusingsimultaneously on rear sight, front sight, and target. Theeye, furthermore, must be in perfect alinement with allthree.5-19. Functions of Reticles.

a. Application. Reticles (para 4-9) are used infire-control instruments for superimposing markings ora predetermined pattern of range and deflectiongraduations on a target. A reticle in its simplest form is apost or picket, or it may consist of two intersecting lines;then the line of sight through their intersection will be inthe center of the field of view. It represents the axis ofthe bore of the weapon when adjusted for short rangefiring or is fixed at a definite angle to the bore of theweapon for long range firing. A reticle is used as areference point for sighting or aiming, or it may be

designed to measure angular distance between twopoints (stadia lines in a transit or grid lines in a militarysight). As it is in the same focal plane as a real image, itappears superimposed on the target and can be viewedby the eye with the same accommodation required forviewing the target or field.

b. Location. In an optical system employing alens erecting system, there are two possible reticlelocations (fig 5-23). It may be placed either in the imageplane of the objective or at the focal point of the eyepiece(image plane of errectors). If the erecting systemincreases the magnification and the reticle is in theimage plane of the objective, the reticle lines will appearwider on the target than if placed at the focal point of theeyepiece. For this reason, the reticle in a low-power riflescope usually is placed in front of the erectors and inhigh-power target-type rifle scopes, it is at the focal pointof the eyepiece. With a lens erecting system, the reticlemay be placed at the front or rear of the erecting system,or reticles may be placed at both points, and theirpatterns are then observed superimposed upon eachother. The preferable position with this type of erectingsystem is in front of the system as the objective andreticle then form a unit and any shift of the erectingsystem does not disturb the alinement of these elements.With a prism erecting system, the reticle usually isplaced in back of the erecting system.

Figure 5-23. Reticle location

5-20. Galilean-Type Rifle Sight. In this British-devised system, a long focal length objective (bearing areticle to its front surface) is mounted in place of an openfront sight. A negative eyelens and aperture replace theconventional peepsight. The aperture provides sharperdefinition and limited parallax by use

of the principle of universal focus through a smallaperture (familiar to most marksmen). In such a system,it is impossible to have both reticle and target image insharp focus simultaneously. Note also that the reticleactually is outside the system. A reticle cannot be placedwithin this system as no real image plane

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exists. Also, the negative eyelens alone would minify thereticle and require eye accommodation to focus on it sothat it would be impossible to see both field and reticle

simultaneously with the same accommodation. Althoughthis system is of basic optical interest, it has no practicalmilitary application in this country.

Section VII. OPTICAL FACTORS IN TELESCOPE DESIGN

5-21. Magnification or Power of Telescope. Powerin a telescope can be determined by dividing thediameter of the entrance pupil by the diameter of the exitpupil.

Power = APEP

where AP is the diameter or aperture of the entrancepupil and EP is the diameter of the exit pupil (fig 5-24).

This formula is applicable to all types of telescopes.

NOTEPower in optical instruments is denotedby the letter x. For example, a 6 x 30binocular is 6-power and has anentrance pupil or objective size of 30millimeters.

Figure 5-24. Entrance and exit pupils.

5-22. Entrance Pupil.a. Function. The entrance pupil diameter is

limited by the clear aperture diameter of the objectivelens. This clear aperture is limited by the inside diameterof either the lens cell or the retaining ring as indicated infigure 5-24. The entrance pupil can be viewed as suchfrom the objective end of the instrument.

b. Measurement. The entrance pupil can beapproximated by measuring directly across the objectivewith a transparent metric scale. Usually the abovemethod is sufficiently accurate for most practicalpurposes.

5-23. Exit Pupil.a. Location. The diameter of the bundle of

light (figs 5-24 and 5-25) leaving an optical system isdetermined by the size of the exit pupil. If an instrumentis held at arm’s length, the exit pupil is seen as a virtualimage of the aperture stop and will appear as a brightdisk of light (fig 5-25). The position of the exit pupil canbe determined by directing the telescope towards anilluminated area, such as the sky, and by holding a pieceof translucent paper or ground glass behind the eyepiecein the place where the emergent beam is smallest andmost clearly defined. This disk of light formed at the exitpupil is called the eye circle or Ramsden circle.

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Figure 5-25. Exit pupil-virtual image of objective.

b. Plane. In figure 5-25, rays 1 and 2originating at opposite limits of the true field (para 5-25)cross at a common point in the objective lens andrecross back of the eyepiece in the plane of the exitpupil. In the exit pupil plane must be the point of rotationof the observer’s eye if the observer is to see the fullfield.

c. Diameter. The diameter of the exit pupil canbe measured by pointing the telescope toward a lightsource (out a window) and inserting a piece of translucentmaterial in the plane of the exit pupil (para 9-3). Thediameter of the image then can be measured on the paper.This diameter also can be approximated by holding thetelescope away from the eye and measuring across theeyelens the diameter of the bright disk seen through theeyelens or it can be measured quite accurately by using adynameter (para 5-24).5-24. Ramsden Dynameter.

a. Basic Use. The Ramsden dynameter(fig 5-26) is used in measuring the position and diameter ofthe exit pupil. The dynameter is essentially a magnifier oreyepiece with fixed reticle. The eyepiece and the reticlemove as a unit within the dynameter tube. When in use,the dynameter is placed between the eye and the eyepieceof the instrument and focused until the bright disk of the exitpupil is sharply defined on the dynameter reticle. Thediameter of the exit pupil is then measured directly on thedynameter reticle (usually graduated in 0.5 millimeters) andthe eye distance or eye relief is read from the scale on thedynameter tube.

Figure 5-26. Ramsden dynameter.

b. Purpose of Measurement. Measurement ofexit pupil and entrance pupil diameters of an instrumentis useful in determining the power of an unknown, or forexample, a foreign instrument by the use of the equation:

Power = AP; (para 5-21). EP

If the power is known and the diameter of the exit pupil isdesired (and a dynameter is unavailable), it is usuallyeasier to measure approximately the entrance pupil andcompute the diameter of the exit pupil.5-25. True and Apparent Fields of Telescope.

a. Definitions. The true field of view in atelescope is the width of the target area or field that canbe viewed. It is expressed as either angular true field orlinear true field (figs 5-27 and 5-28). The former is theangle at the objective included by the two extremeprincipal rays which will enter the observer’s eye. The

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latter is the width of field at 1,000 meters. This is formilitary instruments. Commercial distance is 100 yards

(fig 5-28). The apparent field of view is the angle at theeye included by these two extreme principal rays.

Figure 5-27. Field limit.

Figure 5-28. Linear true field.

b. Determining True Field of Telescope. Truefield can be determined by sighting through theinstrument on some target at the edge of the field. Theangular movement of the instrument required to shift theposition of the target to the opposite edge of the field isthe angular true field of the instrument (para 9-5).

c. Determining Apparent Field of Telescope.The apparent field of the instrument can be measured byrepeating the above procedure while reversing theinstrument. It will be found that the angle required willapproximate the true field multiplied by the power of theinstrument. Threrefore,

Apparent FieldPower = —————————— (a close approximation)

True Field5-26. Factors Determining the Field of View of

Telescope.a. Apertures. The diameter of the effective

aperture of entrance pupil (para 5-21) of the objectivelens is not a determining factor in any telescope exceptin the Galilean system (unsuitable for sighting

instruments). The field of view is limited by the opticalpossibilities of the eyepiece, a consideration of theaperture diameter, and the extent of the apparent fieldfor any given eye relief. Eye relief is the distance fromthe rear surface of the eyelens (para 4-4b ) to the planeof the exit pupil (figs 2-34 and 5-25). The center ofrotation of the observer's eye should be situated in thisplane. For a given eye relief, apparent field (and thustrue field) is related directly to the aperture diameter ofthe eyepiece. For this reason, military telescopicgunsights frequently have huge eyepieces to provide alarge field at the long eye relief required in such aninstrument. In a binocular or observation instrumentdesigned to be used at any eye relief as short as 8millimeters, an eyepiece of small aperture diameter willprovide a large field, when used in a system of suchshort eye relief.

b. Relation to Eye Relief. When eye relief isreduced, the aperture of the eyepiece required to providethe same angle is also reduced. The field of

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view is thus inversely related to eye relief. If the eyerelief is shortened and the aperture of the erectors isincreased accordingly, the field of view will be increasedwithout requiring any increase in eyepiece aperture. Ifthe size of the/eyepiece is increased and erectors are ofcorresponding aperture, the field of view will beincreased for the same eye relief.

NOTECoversion of angular field to linear field:

LF = 2aR, where LF equal Linear Field, R equalsE

Range, "a" equals Angular True Fieldmeasured from the axis, and E equals 1Radian which equals 57.3°(approximately).

5-27. Factors Determining Eye Relief of Telescope.a. Definition. Eye relief is the distance from the rearsurface of the eyelens to the plane of the exit pupil inwhich the eye must be positioned to view the full field(para 2-34). Eye relief can be, and usually is, short inhigh-power observation instruments. In a binocular, it isquite short. Eye relief must be long in rifle scopes orgunsights where recoil is to be considered. Its locationwill depend on the location of the real image(0' in figs 5-29, 5-30 and 5-31) of the aperture stop(objective) formed by the erectors. The closer this is tothe eyepiece the greater is the eye relief.

Figure 5-29. Eye relief-symmetrical erectors with real images at focal points of errectors and image 0’of objective between erectors.

Figure 5-30. Eye relief-objective image outside erectors.

Figure 5-31. Eye relief-collective lens in system.

b. Effects of Field Lenses. A field or collectivelens in the focal plane of the objective will shift this imageforward and shorten the eye relief (the stronger the lensthe farther the shift). For this reason, a collective lensusually is not used in a rifle scope. It

would make the eye relief too short in most cases for therecoil. As mentioned in the note attached to paragraph5-13b, the field lens in the eyepiece shortens eye reliefby shifting the plane of the exit pupil without affecting itssize.

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c. Power of Eyepiece. Eye relief also isinversely related to the power of the eyepiece; i.e.,replacing a low-power eyepiece with one of higher powerwill reduce the eye relief.5-28. Light Transmission or Ilumination.

a. Pupils. The illumination of the imagedepends on the amount of light received by the objectiveand the specific intensity (whether bright daylight ortwilight) of these light rays. The amount of light receivedis determined by the diameter of the entrance pupil (clearaperture, abbreviated as AP) of the objective. Theamount of light entering the eye is limited either by theexit pupil of the instrument or the pupil of the eye,whichever is smaller.

(1) An S x 30 (8-power, 30-mm diameter)glass in daytime (eye pupil diameter of 5 millimeters),since amount of light entering the instrument isproportional to area of entrance aperture and thus to thesquare of its diameter, picks up (30/52) or 36 times asmuch light as the naked eye. The area of the retinalimage in the eye, however, is 82 or 64 times as large aswith the unaided eye. Thus, we have 36 times as muchlight distributed over an area 64 times as great, soillumination on the retina is only 36/64 or 56.3 percent ofthat with the unaided eye.

(2) Consider now an 8 x 40 binocular. Inthis case the amount of light entering the eye is (40/52 or64 times that entering the unaided eye. There is now 64times as much light falling on an area 64 times as large(power is 8, thus the comparative area of the retinalimage is 82 ). In this case, illumination is the same aswith the unaided eye.(3) In an 8 x 50 glass, illumination will not increase with a5-millimeter eye pupil because the iris of the eye will stopand the extra light from the extra 10 millimeters ofdiameter of the objective. Since,

Entrance PupilPower = Exit Pupil (para 5-21).

the exit pupil of this 8 x 50 glass is 50/8 or 6.25millimeters; therefore, the eye pupil of 5 millimeters willnot admit all this light.

b. Maximum Illumination. For maximum

illumination at any given light intensity, exit pupil ofinstrument must equal entrance pupil of eye under thesame condition. Also, with any instrument, retinalillumination never is greater than with the unaided eye.Opaque foreign material such as dust or lint on anyoptical surface, except one in a real image plane, willreduce the illumination of the system.5-29. Night Glasses. When subjected to intenseillumination(brilliant daylight), the entrance pupil of theeye may be stopped down to 2 millimeter diameter.When used under this illumination, an optical instrumentneed have an exit pupil only 2millimeter diameter. Whenthe eye is subjected to very faint illumination, as at night,the pupil opens to a diameter of 8 millimeters. Thus, foruse at night an optical instrument must have an exit pupilof at least 8 millimeters provide the pupil of the eye withall the light it will admit. The relationship between thesize of the exit pupil and the power of an instrument isimportant. For example, a 7 x 50 binocular is suitable fornighttime use because if the clear aperture of theobjective (50) is divided by the power of the binocular (7)(para 5-21), the exit pupil diameter is determined to be7;15 millimeters. This is able to pass approximately allthe light the eye pupil can use at its widest aperture.Now a 7 x 35 binocular provides ample illumination indaytime because its exit pupil diameter is 5 millimeters,which is larger than the eye’s 2-millimeter opening underintense illumination and ample for its 5-millimeteropening under moderate illumination but insufficient forthe 8-millimeter eye opening at night.5-30. Resolving Power of Optical Systems.a. Definition. The numerical measure of ability of anoptical system to distinguish fine detail is called resolvingpower. It is further defined as the reciprocal of thesmallest angular separation (angular limit of resolution)between two points which can be distinguished orresolved as separate points. Resolving power = 1/α,where α = least angular separation in radians (fig 5-32).As the angle becomes smaller between two points thatcan be resolved, the resolving power increases anddefinition, therefore, is better.

Figure 5-32. Angular limit of resolution.

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b. Controlling Factor. The angular limit ofresolution can be proved to be related inversely to thediamter of the lens. According to Dawes’ rule (anapproximation): a (in minutes of arc) = 1/Do (in fifths ofan inch, fig 5-32); thus it is advisable to have large.objectives for sharp definition. The observationtelescope has such an objective of large diameter. Thetargot shooter uses a telescope of 1 1/4-inch diamterobjective or larger. The 7 x 50 (7-power, 50-millimeterdiameter) binocular provides good resolution because ofthe large diameter objective and for this reason is betterin daytime than a 7 x 35.

c. Normal Magnification. Normalmagnification is that power of a telescope giving an exitpupil diameter just equal to that of the eye usuallystandardized at 2 millimeters (Some authorities,however, consider the eye to provide the best resolutionat an aperture of 4 to 5 millimeters.) For example:

D (Objective)Normal magnification = ————————

C (Pupil of Eye)or, normal magnification = 25.4 Do over 2 to 4,

where Do is in inches.Normal magnification is one of three factors limiting themagnifying power obtainable with a given objective lens.The other two factors are the resolving power of thegiven lens (RP = 1/α para 5-30a) based on diffrectioneffects, and the quality of the lens (precision of grindingand polishing and absence of aberrations) measurableby labratory methods (fig 5-32).

d. Other Resolving Factors. The foregoingequations are in agreement if the entrance pupil of theeye is 2 millimeters. The plus 2 to 4 results in emptymagnification, an increase in image size of fine details sothe eye can more readily see them, but without anyincrease in telescopic resolving power. For bestresolution, the power of the eye piece in the system isdetermined by the formula:

Focal Length Eyepiece = Diameter EyeFocal Length Objective

Pupil X Diameter of Objective

NOTEAll values in this formula are in millimeters.Although the resolving power of the human eyenormally is equal to 1 minute of arc, for longcontinued observation this becomes 2 or 3minutes (due to fatigue). This, for continousobservation an instrument of greater power isneeded to provide the same definition obtainablewith a lower power telescope used for shortintervals. Transparent foreign material such asgrease or fingerprints on a lens will impairdefinition (resolving power). Opaque foreignmaterial on the eye-lens either may impairdefinition or blot out small portions of the field.

5-31. Optical Glass Used in the Design of MilitaryInstruments.

a. Common Types. Although there arenumerous types of optical glass in the design of opticalsystems for military instruments, five types in commonuse are as follows:

Index of Critical angleName refraction in degrees

Boro silicate crown 1.5170 41Barium crown 1.5411 40Baryta light flint 1.5880 39Ordinary flint 1.6170 38Dense flint 1.6490 37

It will be noted that as the index of refraction increases,the critical angle decreases.

b. Specific Uses. Boro silicate crown normallyis used for the collective lenses and dense flint for thedispersive lenses in objectives, erecting lenses, andeyepieces. In the Kellner eyepiece (A, fig 4-5), borosilicate crown is used for the field lens, barium crown forthe collective element, and either light flint or ordinaryflint for the dispersive element of the achromatic doubleteyelens. Prisms, generally, are made of boro silicatecrown. For wide-angle instruments, however, baryta lightflint is used. Reticles are usually made of baryta lightflint because it etches best and windows usually are ofboro silicate crown.

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CHAPTER 6

PRINCIPLES OF LASERS

6-1. General. Simply defined, a laser (LightAmplification by Stimulated Emission of Radiation) is alight-emitting body with feedback for amplifying theemitted light. The laser is a unique, and highlyspecialized source of light; the beam of light it produceshas three significant characteristics, namely, laser light ismonochromatic, highly collimated, and coherent.Although light possessing the first two properties can beproduced by some conventional light sources, only laserlight possesses all three properties. In addition, the laserbeam is a powerful and very intense light source.6-2. Types of Lasers.

a. There are essentially four types of lasers:(1) Solid state rod-type lasers use

materials such as a ruby rod of about 1 centimeter indiameter and 15 centimeters long as an elementary lightemitter or generator.

(2) Semiconductor diode-type lasersuse material such as gallium arsenide and consist of ajunction formed by p-type material and an n-typematerial. In this type of laser, stimulation to laseremission occurs by passing a current through thejunction.

(3) Gas-type lasers use helium-neon,argon, carbon dioxide, nitrogen and Xenon. In this typeof laser, stimulation to laser emission occurs by passinga current through the gas. The current causes the gas toionize and radiate. The radiation oscillates within a tubeprovided with mirrored ends and then discharges

from the partially mirrored end of the tube.(4) Liquid-type lasers consist of solutions

such as coumarine and rhodamine red. In this type oflaser, the liquid-laser materials are stimulated toemission by irradiating the lasing liquid or dye solutionwith another laser beam.

b. Although the heart of a laser is a light waveamplifier, the device produces a beam when it oscillates.The laser is, therefore, a special kind of oscillator, but itwill optically behave in the same manner as ordinarylight. For this reason, only gas lasers and solid-staterod-type lasers, are described in detail.6-3. Gas Lasers.

a. A gas laser is structurally a simple device,and in many ways it is similar to neon electric signs. Agas laser (fig 6-1) consists of a thin glass tube about 1foot long, and filled with a low-pressure mixture of heliumand neon gases. A pair of electrodes, a negativecathode and a positive anode, are mounted near theends of the tube. These electrodes are connected to ahigh voltage, direct current power supply. The electricfield produced between the two electrodes breaks downthe column of gas, instantly transforming it from a poorconductor of electricity into a relatively good conductor.A continuous electric glow discharge takes place withinthe glass tube, and produces a continuous electriccurrent flow between cathode and anode through thepartially ionized column of gas.

Figure 6-1. Gas laser.

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b. The tube emits a reddish-orange neon glowbecause the electric discharge causes countlesscollisions among the gas atoms that excite them to highenergy states; the atoms randomly fall back to theirnormal energy state, and, in so doing, emit a bundle oflight energy photons. In a conventional neon sign, theseup and down energy shifts take place at random withinthe glowing gas column.

c. The mixture of gases in the laser tube hasbeen carefully selected so that more neon atoms are inhigh energy states than in low energy states when thedischarge occurs; it is the neon gas that is responsiblefor lasing. That is, as the discharge excites the heliumatoms, they collide with the neon atoms and transferenergy to the neon atoms. This transfer of energy to theneon atoms raises their energy state. Stimulatedemission of photons can now take place within thecolumn of glowing gas. This occurs when an excitedneon atom drops to a lower energy level and emits a lightwave. As the light wave speeds past other excitedatoms, it stimulates them to emit their photons. Thisprocess of amplification continues, since one photonentering the column of gas at one end causes otherphotons to be emitted. The reflecting mirrors at the frontand the rear of the glass tube cause any light wavesemitted in the direction of the tube to bounce

back and forth along the gas column establishing acontinuous lasing action.

d. The electric discharge continuously pumpsthe neon gas atoms to higher energy states, and, at thesame time, light waves bouncing between the mirrorsstimulate the excited atoms into emitting their packets oflight energy. As a result, a steady stream of coherent,monochromatic light is generated within the column. Thelaser tube’s front mirror is designed to be partiallytransmitting, i.e., it is designed to reflect 99% of the lightthat hits it, and allow the other 1% to pass through. Thissmall portion of the laser light generated within the tubepasses through the front mirror as a narrow light beam;its wavelength is 633 nm, and it is deep red in color.6-4. Solid State Rod Lasers.

a. A ruby rod solid state laser consists of anelementary light emmitter or generator that is illuminatedby a high-intensity light (A, fig 6-2). The high-intensitylight, such as that from a photoflash lamp, makes the rodfluoresce with a pink color. The fluorescence persists aslong as the photoflash light persists. This effect is not alaser radiation, but just another optical characteristic ofthe emitter. The ruby rod produces a characteristic pinkcolor because it is made of aluminum oxide (sapphire)containing 0.05 % chromium.

Figure 6-2. Solid-state rod lasers.

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b. In laser radiation, the ends of the ruby rodare highly polished so that light can pass through almostwithout absorption. A mirror is placed at each end andaligned perpendicularly to the principal axis of the rod.When the rod is illuminated with an intense photoflashlight, it emits a fluorescent light which reflects back andforth between the two mirrors. This increase in intensityof the light is produced by the oscillation of the ruby lightwithin an optically resonant cavity formed by the rod andthe two reflecting surfaces of the mirrors.

c. The removal of the laser energy, orradiation, from the resonant cavity is accomplished bymaking the rear reflecting mirror 100% reflective and thefront reflecting mirror partially reflective. This allowssome of the laser light generated within the resonantcavity to pass through as a laser beam of the samediameter as the ruby rod

d. During the optical pumping of the ruby rodby the flashlamp, some of the chromium in the ruby rodbecome excited. This causes their electrons to moveaway from the atoms and position themselves at higherenergy levels from which they spontaneously fall back totheir normal energy states. During this transition, eachof these electrons produces a photon of light. Thesephotons now oscillate by reflecting from one mirrorsurface to the other within the resonant cavity. On theirway to the mirror, some of the photons collide with one ormore atoms which are in an excited state due to theflashlamp pumping, and interact with them to produce aphoton or photons identical in energy and frequency withthe initial photon. The newly formed photons continue tointeract with other excited atoms, producing more

photons, and these photons continue interacting toproduce still more photons. When a threshold energy ofthe total photonic energy within the resonant cavity isattained, a pulse, consisting of a very intense laser beamformed by photon waves, bursts out of the partiallyreflective end of the ruby rod.e. In practical applications the highly polished ends ofthe ruby rod can be mirrored or coated with a dielectricmaterial, such as magnesium fluoride or cerium dioxide(B, fig 6-2). When this is done one end is fully coatedand the other is partially coated so that the emitted laserlight can pass through it. This laser beam, as it emergesfrom the ruby rod, has a slight divergence.6-5. Laser Optics.

a. The extremely tight beam produced by thelaser, with final tightening obtained by external. opticalsystems, gives it the ability to travel extreme distanceswith little divergence. The laser output can be focused toa parallel beam where spreading is expected to be lessthan one foot per mile of travel. A beam of light from aruby laser would suffer so little divergence that, forexample, it would be concentrated in an area ten milesacross when it reached the moon; an ordinary searchlight with the same intensity would place a beam over25,000 miles wide on the moon.

b. Brightness may approach millions of timesthat of the sun, on a relative bandwidth basis. Spectralnarrowness allows good signal to background ratios tobe realized. Compared to microwave systems, laserdevices will allow construction of equipment with anantenna only inches across.

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CHAPTER 7

INFRARED PRINCIPLES

7-1. General. Infrared light is considered to beradiant energy in the band of wavelengths between about0.76 and 100 microns. The portion of the band between0.76 and 3 microns is sometimes referred to as nearinfrared light, and the portion between 3 and 100 micronshas been called far infrared light. Most of the infraredlight band overlaps the heat radiation band ofelectromagnetic radiation. By definition, infrared pertainsto or designates those radiations, such as are emitted bya hot body, with wavelengths just beyond the red end ofthe visible spectrum.7-2. Infrared Radiation.

a. The term infrared radiation is used todescribe electromagnetic radiation whose wave-lengthlies just beyond the red end of the visible spectrum, andthe beginning of the region that can be detected bymicrowave radio techniques. The distinction, forexample, between infrared astronomy and radioastronomy is, therefore, an arbitrary one based entirelyon differences in detection techniques.

b. A study of source characteristics isimportant because of the help such knowledge providesin the choice of detectors and the design of opticalelements. In many applications the radiation source isnot subject to control. This is most often true for passivesystems where the objects are detected by their naturalradiation. It also may be true for those active systemsthat use a source for the illumination of a scene so thatobjects may be detected by the reflected radiation.

c. The natural division or grouping of infraredsources depends upon the nature of the wavelengthdistribution of the emitted energy. One type of sourceemits radiation over a very broad and continuous band ofwavelengths. A plot of its emission versus wavelength isa smooth curve which usually passes through only onemaximum. This type is called a continuous spectrumsource, or simple a continuous source.

d. Another type of source is one whichradiates strongly is some relatively narrow spectralintervals, but, in other wavelength intervals, the sourcedoes not radiate at all. A plot of emission versuswavelength reveals a series of emission bands or lines.The curve is discontinuous and the source is called a

discontinuous spectrum source, a line source, or a bandsource.

e. An intense source of infrared radiation canbe devised using the maser (Microwave Amplification byStimulated Emission of Radiation) principle. This type ofsource is very directional, is concentrated within a narrowspectral interval, and possesses a high degree ofcoherence.7-3. Thermal Radiation. The electromagneticenergy that is emitted from the surface of a heated bodyis called thermal radiation. This radiation consists of acontinuous spectrum of frequencies extending over awide range. The spectral distribution and the amount ofenergy radiated depend chiefly on the temperature of theemitting surface. Careful measurements show that at agiven temperature there is a definite frequency orwavelength at which the radiated power is maximum,although this maximum is very broad. In addition, thefrequency of the maximum is found to vary in directproportion to the absolute temperature. This rule isknown as Wien’s law. At room temperature, forexample, the maximum occurs in the far-infrared regionof the spectrum, and there is no perceptible visibleradiation emitted. But at higher temperatures, themaximum shifts to correspondingly higher frequencies.Thus at about 5000C and above, a body glows visibly.7-4. Infrared Detectors.

a. The central element in any infrareddetection system is the detector. This is the devicewhich transduces the energy of the electromagneticradiation falling upon it into some other form (in mostcases, electrical). The mechanisms used to perform thisfunction are broadly classified as either photon detectorsor thermal detectors.b. When radiation of wavelength less than a criticalvalue falls upon the surface of certain materials,electrons are emitted from the surface. Photo tubesusing this photoemissive effect are very widely used.However, the nature of the effect is such thatphotoemissive detectors operate in the ultraviolet, visible,or very near infrared. The photoemissive effect, alsotermed the external photoelectric effect or simply thephotoelectric effect, was discovered by Hertz in 1887 andexplained by Einstein in 1905. The image converter isthe only type of detector which has found muchapplication in infrared technology, but its use is

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limited to the very near infrared. The most elementalform of photoemissive cell is the vacuum photocell,consisting of a photo cathode and an anode in anevacuated enclosure.c. The photon effect most widely used in infrareddetection is photoconductivity. The reason for this is thatphoton effects in general are faster and more efficientthan thermal effects, and photoconductivity is the mostsimple of the photon effects. Photoconductive detectorsas a class are easier to produce and easier to use thandetectors based upon the other photon effects. Thephotoconductive detector (fig 7-1) is placed in the inputcircuit of an amplifier. In order to detect the

change in conductivity of a photoconductor uponexposure to radiation, it is also necessary to supply abias battery and a load resistor. The photoconductor ofdark resistance R is placed in series with a load resistorRL and a bias battery Vb. Infrared radiation, which ismodulated or chopped, is absorbed by thephotoconductor, thereby causing its resistance todecrease with a corresponding increase in the currentflowing in the circuit and a greater portion of Vb to appearacross RL. The voltage appearing across RL is coupledthrough capacitor c to the input circuit of the amplifier,and represents the radiation absorbed by thephotoconductor.

Figure 7-1. Photoconductive detector.

d. Another internal photo effect of interest inknown as the photovoltaic effect. As the name implies,the action of photons in this case produces a voltagewhich can be detected directly without need for biassupply or load resistor. In addition to the photovoltaicdetector, the operation of the photodiode and thephototransistor are based upon this effect. That is, aslong as radiation falls upon the p-n junctions within thebody of a semiconductor, electronhole pairs will beformed and separated by the internal field at the junction.If the semiconductor ends are short circuited by anexternal conductor, a current flow in the circuit. On theother hand, if the ends are open circuited by a highimpedance voltage measuring device, a voltage will existas long as radiation falls upon the p-n junction.

e. Thermal detectors make use of the heatingeffect of radiation. They are simply energy detectors andtheir responses are dependent upon the radiant

power which they absorb. The most well known forms ofthermal detectors are the bolometer and thethermocouple. The bolometer has seen widespreadmilitary use, while the thermocouple has found extensivecommercial application.

f. A bolometer is a radiant power detectingdevice, the operation of which is based upon measuringthe temperature change in resistance of a material dueto the heating effect of absorbed radiation (fig 7-2). Thesimplest form of bolometer is a short length of fine wirewhose resistance at a given temperature is known.Radiation allowed to fall upon the wire is partiallyabsorbed, causing a rise in temperature. The resistancechange due to the change in temperature is a measureof the radiant power absorbed. Because bolometers arepower detectors, they are capable of detecting radiationof all wavelengths and are widely used as detectors ofmicrowave power.

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Figure 7-2. Bolometer circuit.

g. Bolometers may be of three types: metal,semiconductor, and superconductor. Metal andsemiconductor bolometers are operated at ambienttemperatures, whereas the superconducting bolometermust be cooled to temperatures near absolute zero. Aform of metal bolometer used in microwave workconsisting of an encapsulated platinum wire is known asa barretter; semiconductor bolometers are known asthermistor bolometers, thermistor denoting thermallysensitive resistors. Unlike a barretter the resistance of athermistor drops as its temperature rises. Thischaracteristic has caused semiconductors to be muchmore widely used than metals from bolometers. Asuperconducting bolometer has the advantage overthermistor and metal bolometers of reduced thermalnoise, reduced heat capacity, and a step resistance-temperature curve. On the other hand, the problems

associated with low temperature operation and precisetemperature control are severe.

h. The radiation thermocouple was one of theearliest infrared detectors. It consists of a junction of twodissimilar metals. Absorbed radiation causes thejunction temperature to rise, and the heating of thejunction generates a small flow of electric current. Thisaction is responsible for generating a voltage which isproportional to the temperature rise and, therefore, isproportional to the intensity of the radiation. A widelyused form of the thermocouple is the radiationthermopile; it consists of a parrellel array ofthermocouples. The thermopile has a higher responsethan the thermocouple because of the use of multiplejunctions. However, its response time is long and it is notsuitable for ac amplification techniques. The fragileconstruction makes it of little use in applications where itwould be subject to vibration and shock.

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CHAPTER 8

TYPICAL FIRE CONTROL INSTRUMENTS

Section I. TELESCOPE

8-1. General. Telescope is the standard nomenclaturedesignation for instruments of a number of differenttypes. Some typical telescopes illustrated and describedin this section are designed to be used for observation;others are designed to aim weapons. Of these sometelescopes are of the straight tube type; someobservation telescopes are of the prism-offset type; othertelescopes are of the articulated, elbow and

panoramic types. Telescopes are incorporated in someperiscopes, others are used in constructing binocularsand rangefinders.8-2. Telescope (Rifle).

a. The telescope illustrated in figure 8-1 istypical of those designed for use with the rifle for directfire.

Figure 8-1. Telescope (Rifle)—assembled view, reticle pattern.

b. The telescope is secured in a rear sightmount which is in turn secured to the rifle so that thetelescope and mount move with the rifle in azimuth andelevation. The gunner aims with that part of the reticlewhich represents the desired deflection and range. Thereticle may be illuminated for night operation.

c. This telescope has a slender tube andenlarged eyepiece with a 2.8X magnification andapproximately 70 3 minutes field of view.

d. The telescope body is a piece of cold-

drawn seamless steel tubing about 1 inch in diameterwith an adapter shrunk onto the rear end. The front endof the tube is enlarged to about 1½ inches toaccommodate the objective lens assembly.8-3. Telescope (Sniper’s Sighting Device).

a. This telescope (fig 8-2) is a straight tube-type telescope with a fixed focus and is designed fordirect sighting. It is typical of those designed for use as asniper's sighting device with rifles to aid in obtainingmore accurate fire.

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Figure 8-2. Telescope (Sniper’s Sighting Device)—assembled view.

b. The telescope is characterized by a large bright fieldof view; has an elevation range knob calibrated from 0 to900 meters, in 50-meter intervals, causing a total changein elevation of 12.1 mils. A windage knob, having arange of 20 minutes either right or left, is used to correctdrift caused by the wind. The telescope has amagnification of 2.2 power.c. The telescope is equipped with a sunshade to shadethe objective and prevent reflections and an eyeshield toposition the observer’s eye at the proper eye distance.8-4. Telescope (Observation).

a. The observation telescope (fig 8-3) istypical of those designed for observation purposes by theinfantry in observing the effectiveness of artillery fire.

Figure 8-3. Observation telescope-assembled view withtripod.

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two Porro prisms and magnified by the lenses in theeyepiece, as shown in figure 8-4.

Figure 8-4. Observation telescope-optical elements and optical diagram.

c. The telescope is of 20 x power with a field ofview of 20° 12 minutes, is approximately 14 1/2 incheslong, is focused by turning the knurled focusing sleeveand is mounted on the tripod which supports theinstrument about 11 inches above the ground. With thistripod the telescope can be swung around to any desiredposition. An elevating

screw is provided for elevating or depressing the forwardend of the telescope.

d. The telescope is strapped in position in acradle which in turn is mounted on the correspondingtripod. The tripod folds into a compact unit and fits into acarrying case for ease and convenience intransportation.

Section II. ARTICULATED TELESCOPES8-5. General.

a. The articulated telescope is a component ofthe direct free control system and is generally

mounted coaxially with the gun.b. The optical system diagram of one type of

articulated telescope is illustrated in figure 8-5.

Figure 8-5. Articulated telescope - optical system diagram.

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a. The articulated telescope illustrated in figure8-6 is typical of those designed to supplement theprimary and more sophisticated tank fire control

system. It is the main part of the secondary, direct, firecontrol system used by the gunner and is mountedcoaxially with the gun.

Figure 8-6. Articulated telescope (Tank) - assembled view.

b. The optical system of this articulatedtelescope is shown in figure 8-5. The telescope has an8-power magnification with a field of view of 71/2 and isused for viewing the target during daylight and periods ofartificial illumination.

8-7. Articulated Telescope (Self-PropelledVehicle).

a. The articulated telescope illustrated in figure8-7 and its associated mount are the primary fire controlinstruments for the missile system and the

7.62 mm machinegun, and the secondary fire controlinstruments for conventional ammunition. The telescopeattached to the mount, is coaxially connected with the152 mm gun/launcher and embodies and articulate jointso that the eyepiece remains stationary and available tothe gunner throughout the range of gun elevation anddepression. One of two powers, 8X or 12X, may beselected by the gunner.

Figure 8-7. Articulated telescope ( Self-Propelled Vehicle)-assembled view.

8-4

TM 9-258b. The optical system of this articulated

telescope forms an inverted image of the viewed target.One of the two reticle patterns is projected into theoptical train of the telescope by the beamsplitter andappears superimposed on the

target image. After passing through the prisms in thearticulated joint, the inverted image is erected andprojected by the power lens erecting system through theoffset prisms to the eyepiece.

Section III. ELBOW TELESCOPES8-8. General.

a. The elbow telescope is used wherever it isnecessary to direct the line of sight through a 90� angle.This is desirable where the range of the angle ofelevation of the instrument is considerable. Telescopesof this type are used to permit observers to assume themost convenient positions for the operation of theirinstruments. Such telescopes are employed in thepointing of recoilless rifles, towed and self-propelledhowitzers and guns for tracking the target and fire, andfor other purposes.

b. An elbow telescope is essentially the sameas a straight tube telescope except that an erectingprism (e.g., Amici) may be used in the elbow to erect theimage and also produce the 90� deviation. Thiseliminates the lens erecting system and shortens theoptical system. The schematic diagram of one type ofelbow telescope is illustrated in figure 8-8.

Figure 8-8. Schematic diagram of elbow telescope.

c. There are elbow telescopes of a number ofdesigns differing in components of the optical system,method of mounting, and other details depending uponthe purpose of the instrument. The specific instrumentsdescribed herein are typical of those designed fordifferent purposes.

8-9. Typical Elbow Telescopes.a. The elbow telescope illustrated in figure 8-9

is typical of those designed for direct sighting in elevationas a part of a two-sight, two-man system of the howitzer.It has 3X magnification, 13° 20 minutes field of view, andhas a combination reticle graduated for use with twotypes of ammunition. The elbow telescopes of thisseries are of the fixed focus type, are not provided withfilters, have two reticle illuminating windows, aninstrument light adapter, and are identical except fordifferences in reticle patterns (fig 8-9).

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Figure 8-9. Elbow telescope (Howitzer)- assembled view, reticle patterns.

b. The elbow telescope illustrated in figure 8-10is typical of those designed for use as a component ofvarious sight units for pointing the weapon in azimuthand elevation for both direct and indirect fire. The reticlemay be illuminated for night

operation. It has a 3X magnification and a field of viewof 12° 12 minutes, a removable mounting ring, is of thefixed focus type, and is not provided with filters.

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Figure 8-10. Elbow telscope (Sight Units)-assembledview, reticle pattern.

c. The elbow telescope illustrated in figure 8-11is the basic instrument used for laying the towed howitzerin elevation for direct fire. It is mounted and bore-sightedin a mechanism which is integral with the upper part ofthe elevation quadrant. The instrument is basicallysimilar in function to other direct fire telescopes now inuse except for a reticle presentation of multiple ballisticdata and use of a moveable range marker than can beset to range values for direct fire. This elbow telescopehas a field of view of 142 mils and a 8X magnification.

Figure 8-11. Elbow telescope (Towed Howitzer)-assembled view.

d. The elbow telescope illustrated in figure 8-12is the direct fire telescope used for positioning thehowitzer on targets visible from the vehicle.

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Controls for laying the howitzer on line-of-sight targetsare the same as those utilized for indirect fire, since thetelescope and mounts are bolted to the howitzer mount.Correction for cant is achieved by rotating the telescopewithin its mount and

thereby erecting the reticles with the aid of an illuminatedcross-level. The howitzer is then elevated and the targetalined to the proper range mark on the reticle. It is a 4power elbow-type instrument with a field of view of 10°.

Figure 8-12. Elbow telescope ( Self Propelled Howitzer)- assembled view.

Section IV. PANORAMIC TELESCOPES8-10. General.

a. The panoramic telescope is a type of fire-control instrument employed to aim a gun or howitzer inazimuth. It is used in conjunction with a telescope mountto which it is assembled. The telescope mount travelsin azimuth with the weapon, carrying the line of sight ofthe telescope: The telescope provides the mechanismsfor setting the azimuth angle while other associatedequipment supplies the elevation angle of site (vertical,angle.).

b. The characteristic feature of the panoramictelescope is that it maintains an upright image regardlessof whether the line of sight is directed forward, to theside, or to the rear of the observer (para 4-7d (4) and(5)). The upper or rotating head of the instrument andthe line of sight may be rotated 360� or through anydesired angle in the horizontal plane without requiring theobserver to change his position. The mount provides forthe

raising or lowering of the line of sight to any requiredangle. By the combination of these motions, the line ofsight can be directed on any aiming point.

c. The different types and models of panoramictelescopes are of the same basic design. They differ inthe components of their optical systems, the angle atwhich the eyepiece is mounted with relation to the uprightbody of the instrument, the indexing of the scales,provisions for setting in deflection, method of mounting,reticle patterns, manner of adjustment, lighting, and otherdetails. A number of these differences are determined bythe characteristics of the materiel with which theinstrument is used.

d. An objective prism, a rotating Dove prism,and an Amici prism (fig 8-13) comprise the erectingsystem of the great majority of panoramic telescopes. APechan erecting prism assembly may be used in place ofthe rotating Dove prism.

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Figure 8-13. Panoramic telescope (towed Howitzer) - assembled view, optical elements, and optical diagram.

8-11. Panoramic Telescope (Towed Howitzer).a. The panoramic telescope illustrated in figure

8-13 is typical of those designed for use on a telescopemount primarily to point a gun or howitzer in azimuth forindirect fire and may also be used for direct fire. It has amagnification of 4 power and a field of view of 10°. Anazimuth scale and micrometer are provided for settingdeflections. A throwout lever permits rapid traversingwithout the use of the azimuth knob. The 900 objectiveprism in the head of the telescope may be rotated inelevation a limited amount, if necessary, to bring theaiming point into the field of view.

b. The erecting system is comprised of anobjective prism, rotating Dove prism, and Amici prism(figs 4-11 and 4-22). A plane glass window in therotating head protects the optical system from dirt, dust,and other foreign matter; it particularly protects theobjective prism from scratches and breakage. Acompound objective is mounted between the rotatingprism and the Amici prism. The eyepiece is of theKellner type with an achromatized doublet field lens and

double-convex eyelens. A reticle with appropriatepattern for the type of materiel used is mounted in thefocal plane of the objective between the Amici prism andthe eyepiece. The model designation of the telescopeindicates the reticle pattern. The reticle is illuminated bya lighting system for night use. An open sight on the sideof the rotating head enables the observer speedily to pickup the designated target.

8-12. Panoramic Telescope (Self-PropelledHowitzer).

a. The panoramic telescope illustrated in figure8-14 is the basic instrument used in laying the weapon inazimuth and is mounted directly on the telescope mount.It is a 4 power, fixed-focus telescope with a 170 mil fieldof view. It is equipped with a mechanical counter deviceand a gunner's aid counter is integral with the instrument.Included also is a reset counter which can be set to showa reading 3,200 mils when the telescope is alined withthe aiming stakes and the weapon is parallel to the baseline. A gunner's aid counter mechanism, which permitsazimuth

8-9

TM 9-258corrections for factors peculiar to the individual weaponand its emplacement, can be entered easily into theinstrument and is an integral part of the countermechanism.

Figure 8-14. Panoramic telescope (Self PropelledHowitzer)- assembled view.

b. The 90� head prism, objective lens, reticleand erector lens are included in a single assembly whichpermits no relative movement between the reticle andthe head 90° prism as the cab traverses in azimuth.

Section V. PERISCOPES8-13. General.

a. The basic purpose of the periscope is toraise the line of vision in order that targets may be seenfrom entrenchments, or from behind obstructions, or outof enclosed vehicles. The principle of

operation of the periscope is that double reflection oflight from two paralleled mirrors, each placed at a 45�angle and with their reflecting surfaces facing each other(fig 8-15), forms a normal erect image of the object.

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Figure 8-15. Periscope (Tank Observation)- assembled view, optical elements, and diagram of principle of operation.

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b. Solid 90� glass prisms can be used instead ofplane mirrors (fig 8-16 to reflect the light through

an angle of 90° twice and to displace the line of visionvertically.

Figure 8-16. Optical diagram and optical elements of periscope.

8-14. Periscope (Tank Observation).a. The periscope illustrated in figure 8-15 is

typical of those designed for use as an observationinstrument, to view outside objects from the inside of atank. It has a rectangular body with a removable top orhead and a removable bottom or elbow. The top surfaceof the head and the bottom surface of the elbow slant atthe same 45� angle. Clamps attached to the head andelbow engage latches operated by eccentric assembliesin the periscope body. They hold the head and elbowfirmly on the body yet permit ready removal. Plasticmaterial is used in the construction of the head to permitit to shatter into small fragments

when struck by a projectile and not jam or injure theperiscope body. Spare heads are furnished for areplacement. When the periscope is in place, only thehead projects through the tank.

b. The optical system (fig 8-15) consists of twoplane mirrors, two vertical windows, and two horizontalwindows. The mirrors are mounted at 450� angles,parallel to and facing each other. One mirror is mountedin the head, the other in the elbow. The vertical windowsare mounted in the front of the head and in the rear ofthe elbow. The horizontal windows, of thicker glass, aremounted in the bottom of the head and in the top of theelbow. Gaskets around the edges of the

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windows provide a seal against the entrance of dust andmoisture. Crosslines are etched on the horizontalwindows for rough sighting in observation.

c. The periscope is supported in either aviewing or a retracted position in a periscope holderwhich is secured to the body of the vehicle. The holderpermits rotation of the periscope in elevation andazimuth.

d. This periscope is 11 inches long, 6 1/2inches wide, 3-1/8 inches in thickness, and has an offsetof 8-7/8 inches.

8-15. Periscope (Self-Propelled Vehicle).a. The periscope illustrated in figure 8-17 is a

unity-power daylight viewing device employed by thedriver of the vehicle. Three periscope stations areprovided within a notable armored hatch which afford thedriver the maximum of safety and vision of the terrainduring tactical maneuvers.

Figure 8-17. Periscope (Self-PropelledVehicle)assembled view.

d. The head assembly contains a 90� prism fordirecting the forward line of sight through the window ofthe body assembly. The body assembly contains theaforementioned window and another 90° prism thatdisplays the field of view to the driver. This periscopeprovides a 500 horizontal and 140 vertical field of view.

8-16. Periscope (Tank).a. The periscope illustrated in figure 8-18 is a

monocular-type optical sighting instrument that serves asthe main component in the fire control system of themachinegun on the tank.

Figure 8-18. Periscope (Tank)- assembled view

b. Each periscope consists of two separateunits, a head assembly and a body assembly. Both arecoupled together by a common mount.

c. Two 90� prisms and an optical instrumentwindow (fig 8-16) comprise the optical components ofthis periscope.

b. The eyepiece of the periscope, whichcontains the eyelens, center lens and field lens (fig 8-19)is fixed and remains stationary relative to the vehicle.The prism rotates the line of sight from 15� depression to60� elevation in maintaining the line of sight parallel tothe line of fire of the machinegun throughout the gun'sfull range. Connection from the gun to the periscope ismade

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through a quick-release sight link assembly, movementof which is transmitted to the prism through tape-connected pulleys. The sight link

assembly is not part of the tank periscope, but is issuedas equipment with the periscope.

Figure 8-19. Periscope (Tank) - diagram of optical system.

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c. Means for making azimuth boresightingadjustment is provided by azimuth adjustment pin at themounting flange of the sight. Elevation adjustment isprovided at the connecting arm to which the sight linkassembly connects. A rubber eyeshield, mounted overthe periscope’s eyepiece, serves to protect the eyelensand prevent injury to the machinegunner.

8-17. Periscope (Infrared).a. The periscope illustrated in figure 8-20 is an

infrared viewing device of the binocular type used in nightdriving of vehicles. Invisible infrared rays are projectedforward from headlamps at the bow of the vehicle to"illuminate" the field of view. The periscope converts theinfrared image to a visible image which is viewed throughconventional eyelenses. For best vision at averagespeeds the range of the periscope is focused at 18yards.

Figure 8-20. Periscope ( Infrared) - assembled view.b. This periscope has a magnification of 1

power, a field of view of 26.8° and a focal point of18 to 20 yards.

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Section VI. BINOCULARS8-18. General.

a. A military binocular or field glass consists oftwo terrestrial prism offset-type telescopes pivotedabout a hinge which provides adjustment forinterpupillary distance for use with both eyes. The lefttelescope of many models contains a reticle (H and I, fig4-25). An erecting system is required in each of thehalves of the instrument; prisms are used as erectors toincrease stereoscopic

vision and to provide a compact instrument (fig 8-21).The optical axis of each scope must be parallel to thehinge throughout the entire movement of theinterpupillary range; otherwise each eye will see adifferent field resulting in a double image, if deviation isgreat, and headache will result from eyestrain in bringingthe two fields into coincidence.

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Figure 8-21. Binocular (General Use)- assembled view, optical elements, and optical diagram.

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b. The power or magnification of the binocular,like that of the monocular telescope, depends upon thefocal lengths of the objective and eyepiece groups. Thetrue field of view depends upon the design of the lensesand the power. The brightness of image depends uponthe size of the objective. The amount of the exit pupil thatcan be used depends upon the size of the pupil of theeye of the observer which varies from about 2 millimetersfor very brilliant illumination to about 4 or 5 millimeters indaytime, and about 8 millimeters for very faintillumination. The design of an instrument of this typedetermines its suitability for a specific purpose. Abinocular with large objectives and exit pupils is generallybetter suited for observation at night and other conditionsof poor visibility. A binocular of this type is often referredto as a "night glass."

c. Binoculars usually are designated by thepower of magnification and the diameter of theobjectives. Thus, a 6 x 30 binocular magnifies 6diameters and has objectives which are 30 millimeters indiameter. This designation usually is stamped on theinstrument.

d. Binoculars permit the use of and increasethe radius of stereoscopic vision. The observer viewsthe object from the two objectives which are more widelyseparated than his eyes, while the magnificationprovided by the instrument increases his range of vision.For example, if the distance between the lines of sight ofhis eyes is doubled by the use of prism binoculars and a6-power instrument is used, the radius of stereovision isincreased from a normal of approximately 500 meters toapproximately 6,000 meters (500 meters times 2 times6).

e. The binocular hinge is equipped with a scalewhich indicates in millimeters the interpupillary distance(distance between the pupils of the eyes). When theproper setting for the observer has been determined, anybinocular may be adjusted at once for the correctinterpupillary distance.

f.. All of the binoculars covered in this manualare constructed for separate focusing; that is, each

eyepiece can be focused independently of the other byturning the diopter scale in a plus or minus direction.The scale, which is calibrated in diopters, indicates thecorrection required for the corresponding eye. Once theproper setting for diopter adjustment has beendetermined for each eye, the setting may be appliedimmediately on future use.

8-19. Binocular (General Use).a. The binocular illustrated in figure 8-21 is

typical of those designed for use as a general purposeinstrument by all services for observation andapproximate measurement of small angles. It has amagnification of 6 power, the objective diameter is 30millimeters, and the field of view is 80 30 minutes. Eachof. the prism offset-type telescopes has an achromaticobjective and a Kellner-type eyepiece with a compoundeyelens and a plano-convex field lens. A porro prismerecting system is employed in each telescope. Theeyepieces are individually adjustable from plus 4 tominus 4 diopters to meet eyesight variations. Graduateddiopter scales permit prefocusing of both eyepieces.The interpupillary distance adjustment also is providedwith graduations to permit presetting.

b. A reticle is included in the optical system forthe left eye (I, fig 4-25). The binocular is equipped with afilter and a carrying strap. This model has improvedwaterproofing.

8-20. Battery Commander’s Periscope.a. The battery commander’s periscope

illustrated in figure 8-22 is a binocular instrument whichconsists of two periscope-type telescopes joined at thetop by a hinge mechanism, to permit adjustment of thedistance between the eyepieces. The eyepiece beingabout 12 inches below the line of sight. The BCperiscope is not hinged at the bottom (as were earliermodel BC’s periscopes) and is typical of those designedto be used only for periscopic observation, with thetelescopes in a vertical position.

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Figure 8-22. Battery commander’s periscope - assembled view, reticle pattern.

b. The BC periscope employs 90� prisms at thetop with their mates at the bottom to complete a Porroprism erecting system (para 4-5c) (thus eliminating thelens erecting system) and also to effect a verticaldisplacement of the line of sight.

c. The instrument has a magnification of 10power and a field of view of 60. It is used for observationand for measuring angles. The right telescope containsa reticle, calibrated in mils (1/6400 part of a circle), formeasuring angles in

elevation and azimuth. One lamp provides illuminationfor the reticle. Another lamp, on a flexible cable, is usedas a hand light to read the instrument scales andmicrometers.

d. Each periscope is provided with filters;amber, red, neutral (smoke), or a clear window may bemoved into position between the objective lens andeyepiece of either telescope.

Section VII. RANGE FINDERS8-21. General.

a. The range finder is essentially two periscopictelescopes or sights (either with one eyepiece or two)with two lines of sight through prisms or reflectors atends converging on the target. The

angle of convergence is indicative of the range. Thecompensator lenses (27 and 28, fig 8-23) provideangular deviation of the right line of sight for ranging.

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*PART OF BONDED PORRO REFLECTOR ASSEMBLY

LEFT END HOUSING ASSEMBLY OPTICAL COMPONENTS1. End housing window 12. Right coincidence reticle window2. End housing penta reflector 13. Collimator lens3. End housing correction wedge 14. Right coincidence reticle4. Range finder end window 15. First collective lens5. Filter 16. Main boresight (gunlaying) reticle6. Porro reflector (part of bonded porro reflector assembly) 17. Second collective lens7. Porro reflector (part of bonded porro reflector assembly) 18. First erector lens8. Objective lens 19. Correction wedge9. Corrective wedge 20. Correction wedge10. Corrective wedge 21. Correction wedge.11. Right reticle lamp

RIGHT END HOUSING ASSEMBLY OPTICAL COMPONENTS22. End housing window23. End housing penta reflector24. End housing correction wedge

RIGHT MAIN HOUSING ASSEMBLY OPTICAL COMPONENTS25. Range finder end window 41. Beam splitter prism26. Filter 42. Eyepiece field stop27. Compensator lens 43. Field lens28. Compensator lens 44. Eye lens29. Range scale prism 45. Combining prism30. Porro reflector (part of bonded porro reflector assembly) 46. Left ocular prism31. Porro reflector (part of bonded porro reflector assembly) 47. Left reticle lamp32. Prism 48. Left coincidence reticle window33. Objective lens 49. Left coincidence reticle34. Porro prism 50. First 90-degree prism35. Reticle lens 51. Second erector lens36. ICS wedge 52. Second 90-degree prism37. Reticle lens 53. Reticle lamp38. Correction wedge 54. Diffusion disk39. Collimator lens 55. Auxiliary boresight (gunlaying) reticle40. Right ocular prism 56. Reticle mirror

Figure 8-23. Optical system schematic.

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b. The purpose of a range finder is to find therange of an object or target. This instrument measuresdistance by triangulation.

c. The range finder contains two opticalsystems which permit a single observer to view the targetfrom points some distance apart. This distance servesas the known leg of the triangle. It is termed the baseline (fig 8-24) and is one form of designation of a rangefinder, that is, a 1-meter base instrument. One of thetwo angles from which the target is observed is fixed at900; the other

angle is variable, depending upon the distance of thetarget from the instrument. The angle to which the 90�and the variable angle converge is termed the parallacticangle. It becomes larger as the distance to the observedobject becomes less. Because of the extreme shortnessof the base line in comparison with the range to becomputed, the utmost accuracy and precision arerequired in the determination of the angles at the twoends of the base line.

Figure 8-24. Fundamental triangle of range finder.

d. In the stereoscopic range finder, by turningthe range knob, the target is made to appear in the samedistance plane as the central measuring mark (pip) of thestereo reticle, when "stereoscopic contact" isestablished.

e. In the coincidence range finder, the twooptical systems are focused into a single eyepieceassembly. When the range knob is rotated, the left andright fields of -view are superimposed and "coincidence"of the two images of the selected target is established.

f. A very simple and practical type ofstereoscopic range finder, based on still anotherprinciple, consists of a binocular telescope having ascale in each ocular. The two scales will fuse into asingle scale, in relief, having the appearance -f a row ofdots receding to infinity. These dots are seen alongsidethe scene observed and every object in the sceneappears to be located in the same plane as one of thedots. Distance to rapidly moving objects can bedetermined approximately very quickly by this method,while a slower

method could not be used to advantage with a veryrapidly moving target.

8-22. Range Finder, Tank (Typical CoincidenceType).

a. General. The range finder illustrated infigure 8-25 is typical of the coincidence-type instrumentsdesigned for use as the principal ranging device in theprimary sighting system of tanks, and is used for directfire operation. This range finder is designed to provideautomatic and continuous ranging information through anoutput shaft to the ballistic computer. For sightingsystems that do not include a ballistic computer, rangeinformation may be read from the range scale. Accuracyof the range finder is inherently high because of its longbase length, 79 in., and 10-power magnification. Therange finder may also be used, if the gunner’s periscopeis out of commission, for boresighting the gun. Either theleft or right end of the range finder may be used forboresighting.

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Figure 8-25. Range finder , tank (typical coincidence type).

b. Characteristics. This typical coincidence-type range finder measures 85 x 14 x 12 inches overall,weighs 149 pounds including end housing assemblies,has a range of 500 to 4400 meters, and a field of view of4°.

NOTEThe key letters shown in parenthesisin c and d below, refer to figure 8-23.c. Theory of Operation.(1) Light from a target enters the range finder

through the right and left end housings. There is adefinite parallactic (convergent) angle between the lightentering through one end housing window from the targetand light entering through the other end housing windowfrom the target. This parallactic angle varies with targetdistance, being larger for close targets than for distanttargets. The optical system (fig 8-23i utilizes thisparallactic angle variation as a means of measuringtarget range. This is accomplished by displacing aspecially designed lens a calibrated distance in order todeviate the light through the parallactic angle andestablish coincidence of the target at the eyepiece.

(2) Light rays, upon entering each housingwindow, are reflected 90 degrees toward the center ofthe range finder. These light rays enter the telescopeoptical systems and, after passing through the lensesand being reflected by the prisms, leave the instrumentby way of the single eyepiece. Light rays from theinternal collimator optical system are projected to thetelescope optical systems by means of reflectors.Images formed at the eyepiece diaphragm are acomposite of a target image and the image of theinternal collimator optical system coincidence reticles forboth the right and left side optical systems.

d. Functional Description.(1) General. For purposes of description, the

left optical system, the right optical system, reticle

patterns, and reticle illumination system of the rangefinder are treated separately. Component parts of eachsystem are individually described to define theirfunctional purpose.

NOTEThe key numbers shown in parenthesesrefer to figure 8-23.

(2) Left optical system. The left opticalsystem consists of an end housing assembly, a mainsighting system, and a collimating system. A targetimage picked up at the end housing assembly istransmitted into the main sighting system. A coincidencereticle image projected from the collimating system isalso projected in this main sighting system. Theseimages merge with the gunlaying reticle and aretransmitted to the eyepiece which is common to both theleft and right optical systems.

(a) End housing assembly. The target image ispicked up by the end housing assembly and turned 90degrees for introduction into the left main sightingsystem.

1. The end housing window (1) is a plano plate,that seals the end housing against dirt and moisture.

2. The end housing penta reflector (2) providesa constant deviation of 90 degrees. The penta reflectoris adjustable to correct for tilt and deviation.

3. The end housing corrections wedge (3) onthe inboard end of the housing may be used to correctdeviation due to initial misalignment of the penta reflectoror housing. The wedge also serves as a window, whichseals the end housing against dirt and moisture.

4. The end face of the housing casting isprecision machined. Alinement keys are used to positionthe end housings to the main housings.

(b) Left main sighting system. The left mainsighting system carries the target image from

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the left end housing assembly to the eyepiece. The leftmain sighting system is housed in both left and rightmain housing castings.

1. The range finder end window (4) is a planoplate, which seals the left sighting system against dirtand moisture.

2. The filter (5) increases contrast between thetarget image and the illuminated coincidence reticle (49).A matched filter (26) for the right main sighting system isengaged simultaneously with the left filter by a controllever on the right main housing.

3. A porro reflector assembly (6) and (7)consisting of a partial reflector (7) and a full reflector (6)bends the coincidence reticle image 180 degrees tosuperimpose it on the target image. The partial reflector,in the line of sight of the main sighting system, picks upthe coincidence reticle image from the full reflector,which picks up the image from the left collimatingsystem. The path of the target image is not affected bythe partial reflector. Approximately 65 percent of theincoming field light is transmitted through the partialreflector, 30 percent is lost by reflection and 5 percent islost by absorption. Less than 30 percent of the light fromthe collimating system is transmitted into the line of sight;70 percent of the light passes through the partial reflectorand is lost. However, collimator light intensity can bevaried by a rheostat.

4. The left objective lens (8) receives light fromthe target and coincidence reticle, and provides a focusadjustment. Light rays from the left objective lens (8) areconverged upon the first collective lens (15) and arebrought into focus at the main boresight (gunlaying)reticle (16). Moving the collective lens adjusts the focallength to the’ desired angular values of the reticlecalibrations. The left objective lens and the first collectivelens function together as a single lens.

5. The main boresight reticle assemblycontains the main boresight (gunlaying) reticle (16).Because it is in the focal plane of the objective lens (8)and the first collective lens (15), this reticle issuperimposed on the target image. Control knobs movethe boresight reticle vertically and horizontally to aline itwith the target image for boresighting, range estimation,or both. The main boresight reticle assembly has acollective lens (15 and 17) on each side of the reticle(16) to increase the field or peripheral light.

6. A composite image of the reticles and target,formed by the second collective lens (17), is located atthe focal plane of the first erector lens (18).

7. The target and reticle images are positionedvertically and horizontally by a series of

three correction wedges (19, 20, and 21). The verticalcalibration mechanism optical components consist of twocorrection wedges (19 and 20). A gear located betweenthe two wedges causes these wedges to rotate inopposite directions. Light rays entering the firstcorrection wedge (19) are bent towards the thickest partof this wedge. Due to the relationship of the thicksections, a vertical vector resultant is formed whichoptically raises or lowers the light rays. The horizontalcalibration mechanism consists of a single correctionwedge (21). This correction wedge receives the lightrays from the second correction wedge (20) of thevertical calibration mechanism. As the horizontal wedgeis rotated, light rays are deviated in a horizontal direction.

8. The first 90-degree prism (50) reflects thetarget and reticle images into the second erector lens(51).

9. The second erector lens is used to set thefocus during the assembly of the range finder, and tobring the focal plane of the left main sighting system tothe eyepiece field stop (42).

10. The second 90-degree prism ’52) bends thelight rays into the left ocular prism (46).

11. The left ocular prism (46) is a penta prismthat reflects the target image 90 degrees into thecombining prism (45).

12. This combining prism displaces the light raysparallel to itself. The light rays are then reflected into theeyepiece assembly by the rhomboidal prism componentof the combining prism.

(c) Eyepiece assembly. The eyepiececontains a field stop and an eyepiece assembly. Thefield stop (42) is a diaphragm for the eyepiece assembly.It restricts extraneous or stray light for a sharp circularfield of view. The eyepiece assembly has a field lens(43) and an eye lens (44), which together magnify theimage.

(d) Left collimating system. The left collimatingsystem projects the left coincidence reticle into the leftmain sighting system so that the coincidence reticleappears in the field of view.

1. The left coincidence reticle (49) isilluminated and projected into the collimator lens (13).

2. The collimator lens (13) gathers thediverging light from the reticle assembly and directs it,with the coincidence reticle image, into the porro reflectorassembly (6 and 7).

3. The collimator assembly contains twocorrection wedge cell assemblies, each containing awedge (9 and 10) that alines the coincidence reticle indeflection and elevation so that it is projected in an exactrelationship to the target image as seen in the mainsighting system.

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TM 9-258(3) Right Optical System. The right optical

system consists of an end housing assembly, a mainsighting system, and a collimating system. A targetimage picked up at the end housing assembly istransmitted into the main sighting system. A coincidencereticle (14), projected from the collimating system, is alsoprojected in this main sighting system. An auxiliarygunlaying reticle is located in this system, and, ifilluminated, is projected into the main sighting system.These images are transmitted to the eyepiece which iscommon to the left and right optical systems.

(a) End housing assembly. The end housingassembly for the right optical system is identical to thatfor the left optical system in (2) (a) above.

(b) Right main sighting system. The right mainsighting system carries the target image from the rightend housing assembly to the eyepiece. The right mainsighting system is housed in the right main housingcasting.

1. The range finder end window (25) is a planoplate and seals the right sighting system against dirt andmoisture.

2. The filter (26) increases contrast betweenthe target image and the illuminated coincidence reticle(14). A matched filter (5) for the left main sightingsystem is engaged simultaneously with the right filter bya control lever on the right main housing casting.

3. The compensator lenses (27 and 28)provide angular deviation for ranging.

4. The porro reflector assembly (30 and 31) inthe right main sighting system is similar to that of the leftmain sighting system, except for the halving adjustment.The halving adjustment tilts the porro reflector to changethe elevation of the coincident reticle (14) in relation tothe target image.

5. The 90-degree prism (32) bends the targetimage into the objective lens (33).

6. The objective lens (33) converges the targetimage and the coincidence reticle image to a focal pointat the eyepiece field stop (42) through the beam-splitter,the, right ocular,, and the combining prisms.

7. The beam-splitter prism (41) reflects theimage 90 degrees into the right ocular prism (40) andsuperimposes the image of the auxiliary boresight reticle(55) on the target image. It also dissipates part of thelight in the right main sighting system to balance thetransmitted light with that in the left main sighting system.

8. The right ocular prism (40) reflects the targetimage into the right angle prism component of thecombining prism (45). At this

point, the light combines with the reflected light from theleft field of view and proceeds to the single eyepiece.

(c) Right collimating system. The rightcollimating system performs the same function as the leftcollimating system in (2) (d) above.

1. The right coincidence reticle (14) isilluminated and projected into the collimator lens (39).

2. The collimator lens (39) gathers thediverging light from the reticle assembly and directs itwith the coincidence reticle image into the porro reflectorassembly (30 and 31).

3. The ICS corrections wedge mount assemblyfor the right collimating system is similar to the collimatorassembly for the left collimating system, except that theICS wedge (36) for deflection correction can be rotatedby the ICS knob. This moves the coincidence reticle (14)in a horizontal plane.

(d) Auxiliary boresight (gunlaying) reticlesystem. The auxiliary boresight (gunlaying) reticlesystem can be used for auxiliary sighting and rangeestimation if the left optical system becomes inoperative.It can also be used to check range finder alinement bychecking its coincidence with the left main boresightreticle (16). The system consists of two assemblies: anauxiliary boresight (gunlaying) reticle bracket assemblyand an auxiliary boresight (gunlaying) lens assembly.

1. The auxiliary boresight (gunlaying) reticlebracket assembly has a reticle lamp (53), a dark fieldcemented auxiliary boresight (gunlaying) reticle (55), andreticle mirror (56) to reflect the reticle image into thesystem.

2. The auxiliary boresight reticle lens assemblyhas two reticle lenses (35 and 37). Both reticle lensesare adjustable longitudinally along the optical axis forproper focusing of the reticle image at the eyepiece fieldstop. In addition, reticle lens (37) is adjustabletransversely by means of the auxiliary boresight knobs toprovide horizontal and vertical movement of the reticle inthe field of the right optical system.

3. The porro prism (34) turns the light rays 180degrees into the beam-splitter prism (41).

(4) Reticle Patterns.(a) General. Four reticle patterns are used in

the range finder. There is one pattern for gunlaying (A,fig 8-26), one for auxiliary gunlaying (B, fig 8-26), andtwo-coincidence reticle patterns (C, fig 8-26). Thecoincidence reticles are provided to aline the opticalsystems (left and right) to each other. An auxiliary reticleis projected into the right optical system in the event ofdamage to the left optical system.

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Figure 8-26. Range finder reticle patterns.

(b) Coincidence reticle. The rightcoincidence reticle (14, fig 8-23) in the right opticalsystem is composed of a vertical line located on thevertical axis of the coincidence reticle pattern (C, fig 8-26) and a horizontal line located at the top and to the leftof the vertical line. This pattern forms the lower verticaland left horizontal bars of the coincidence reticle pattern.The left coincidence reticle (49, fig 8-23) is located in thecoincidence cell assembly of the right main housing. Theleft coincidence reticle pattern (C, fig 8-26) is formed by avertical line located on a vertical axis of the reticle diskand a horizontal line located at the bottom and to theright of the vertical line. This

pattern forms the upper vertical and right horizontalmembers of the coincidence reticle pattern. The right halfof the coincidence reticle is alined with the left half of thecoincidence reticle to establish a reference calibration ofthe instrument when the target image is brought intocoincidence.

(c) Main boresight (gunlaying) reticle. Themain boresight (gunlaying) reticle (16, fig 8-23) is locatedin the focal plane of the objective lens system, whichresults in the target image and the reticle image(B, fig 8-26) being superimposed. The aiming point of thepattern is a boresight cross, 2 mils x 2 mils, coincidingwith the intersection of the vertical and horizontalgeometric

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axes. Two lines and two spaces, located on thehorizontal axis on both sides of the boresight crossalong the horizontal axis, provide a zero elevation andlead reference representing a field 40 mils in width. Thespaces located immediately adjacent to each 1-mil armof the boresight cross are 4 mils wide. Each line andspace located beyond the 4-mil space represents 5mils. Elevation and depression reference is provided bya pair of symmetrically placed broken lines of 2 milsbelow the center of the boresight cross. Each of theselines consist of a line, a space, and a line of 5 milseach. A 1-mil space and a 2-mil line arranged aboveand below the ends of the vertical member of theboresight cross on the vertical axis provide a zeroreference in azimuth.

(d) Auxiliary boresight (gunlaying) reticle.

The auxiliary boresight (gunlaying) reticle pattern (A, fig8-26) is identical to the main boresight (gunlaying)reticle pattern ((4) (c) above). The reticle disk for this

reticle is coated to prevent light rays from passingthrough the disk except at the lines of the reticlepattern.

(5) Electrical lighting system. A 24-volt dc electricalsystem provides illumination for the reticle and therange scale. A control panel on the back of the rangefinder provides controls for light intensity of the reticlesand scales.

e. Metric System. Since range distances are beingconverted to the metric system of measurement ratherthan yards, future range finders will measure distancein meters.

8-23. Tabulated Data. The data listed in table 8-1includes basic general characteristics of a typical rangefinder which are necessary to Army maintenancepersonnel for performing their function.

Table 8-1. General Dimensions and Leading Particulars

Technical data Value

Length (overall) .................................................... 84 in.Height (overall)..................................................... 12 in.Depth (overall) ..................................................... 14 in.Weight (including end housing assemblies) ........ 149 lbBase length.......................................................... 79 in.Range .................................................................. 500 to 4,400 metersMagnification........................................................ 10 powerField of view......................................................... 40Exit pupil .............................................................. 0.120 in.Diopter scale........................................................ 4.00 diopters

Section VIII. SPECIALIZED MILITARY OPTICAL INSTRUMENTS

8-24. Collimator Sight.a. The collimator sight (fig 8-27) is an ingenious

and relatively inexpensive type of sighting device. Thecollimator is inferior to the telescope in effectiveness or

convenience of operation but its simplicity, ruggedness,and low cost make it particularly desirable as a sightingdevice for mortars.

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Figure 8-27. Collimator sight-optical system and optical diagram

b. The principle of the collimator is that a reticle,fairly close to the observer, can be optically transferred toa position infinitely distant from the observer. Parallaxbetween the reticle and the target is thereby eliminated.Since there are only two optical elements, a reticle on aground glass window and an eyelens, the entire structurecan be housed in a compact tube. The eyelens is calleda collimating lens because it renders rays parallel. Itpermits observation of the reticle at infinity or with thesame eye accommodation as required to view the target.The ground glass window is covered inside with anopaque coating, excepting an uncoated center cross orvertical line, through which diffused light can enter to beviewed

through the eyelens as a cross or vertical line of light.This cross or line of light is the reticle which is placed inthe principal focal plane of the eyelens for the infinityadjustment.

c. When employed as the sighting device for aweapon, the mount of the collimator is provided withleveling mechanisms and scales which permit theweapon to be placed at a prescribed elevation withrelation to the line of sight established by the collimator.Sighting is accomplished by looking into the collimatorand at the target simultaneously or successively tosuper-impose the cross or line upon the target.8-25. Reflector (Reflex) Sight. A typical opticalsystem of this kind is illustrated in figure 8-28.

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Figure 8-28. Reflex sight.

a. Condensing Lens. Light from a lamp or reflectedsunlight is concentrated on the reticle (para 4-9), usuallyby using a condensing lens or lens system. A frostedsurface (either one surface of the condensing lens or ofa plane glass plate) may be used to diffuse this light andobtain even illumination of the reticle. This light thenpasses through either a perforated reticle (usuallypunched in a metal disk) or illuminates a reticle patternetched on glass.

b. Collimating Lens. Light from the illuminatedreticle passes through a collimating lens placed one focallength from the reticle. The collimated light from thereticle is ordinarily introduced into the field of view by theuse of a high-reflectance-coated glass plate whichpermits light to pass straight through from one side butreflects most of the light coming from the oppositedirection. A half-silvered mirror may perform the samefunction but less efficiently.

c. Parallax. To remove parallax from this type ofoptical system, either the collimating lens or the reticlemust be moved until the reticle pattern .s clearly definedor sharply focused when viewed through the collimatingtelescope.

d. Operation. Since the reflected light rays from

the image of the reticle are parallel, due to the action ofthe collimating lens, the reticle pattern appears at infinitywhich makes it possible for the observer to super-imposethe image on the target and focus on both at once.

e. Use. The reflex sight is a projection-typecollimator sight used for direct sighting of machine guns.8-26. Aiming Circle.

a. The aiming circle (fig 8-29) is used to measurethe azimuth and elevation bearing angles of a ground oraerial target with respect to a preselected base line. Theaiming circle has many of the characteristics of asurveyor’s transit. Basically, it consists of a telescopemounted on a mechanism which permits unlimitedazimuth and limited elevation movements. By rotatingtwo orientating knobs, zero azimuth heading with respectto magnetic north or any other selected compassheading can be established. The azimuth orientingcontrol knobs can be disengaged for rapid movement byexerting an outward pressure on the knobs. Themechanisms are spring-loaded and will reengage whenoutward pressure is removed. A locking device securesthe compass after the orienting adjustment, has beenmade.

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Figure 8-29. Aiming circle.

8-27. Laser Range Finder. The laser range findershown in figure 8-30 is a laser electronic device whosefunction is to improve the first-round hit capability of theprimary weapon. This function is

accomplished by transmitting a pulse of laser light,receiving reflecting light from the target, and convertingthe time from transmission to reception into range data.

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1. Display Unit2. Control Unit3. Electronics Unit4. Receive/Transmit Sight Unit5. Auxiliary Power Supply Unit

Figure 8-30. Laser rangefinder.

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8-28. Helmet Directed Subsystem.a. The fire control subsystem shown in figure 8-31

is a helmet-directed sighting subsystem. Thissubsystem, which is helicopter mounted, interfaces

with the gun turret and the telescope sight unit of theTOW (tube-launched, optically-sighted, wire-guided)missile subsystem.

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Figure 8-31. Fire control subsystem, helmet-directed.

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Legend for figure 8-31:1. Reflex sight M732. Pilot helmet sight (including pilot linkage and

extension cable)3. Electronic interface4. Gunner helmet sight including gunner linkage

and extension cable)

b. The system enables the helicopter pilot andcopilot/gunner to rapidly acquire visible targets and todirect either the gun turret or the sight unit to thosetargets. The helmet-mounted optical sight

extends over the operator’s right eye, and an illuminatedreticle pattern is projected into the optical sight.Electromechanical linkages sense the helmet sight linesand generate sight-line signals which are processed bythe electronic interface assembly. Either operator cancommand the gun turret or sight unit by means ofoperator-select able cockpit switches. Under certaincircumstances, the turret and sight unit can becommanded simultaneously, one by each operator.

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CHAPTER 9

TESTING OPTICAL PROPERTIES OF INSTRUMENTS

9-1. General. This chapter describes a number ofrelatively simple procedures by means of which theproperties and qualities of complete instruments may bemeasured or calculated. For the most part, the tests canbe made without special equipment. Laboratory testmethods are purposely avoided.9-2. Objective Aperture. The objective aperture size islimited by the clear area provided by the mount of theobjective. It can be measured in millimeters or decimalparts or fractions of an inch with a scale, rule, or with aRamsden dynameter (para 5-24).9-3. Measurement of Exit Pupil and Eye Distance.Direct the instrument at an illuminated area. Focus theexit pupil (fig 2-54) on translucent paper or ground glass.To focus the exit pupil, move the paper or glass towardsor away from the eyepiece until the exit pupil is seen inits smallest diameter as a sharp circular shape of evenbrightness. Measure the distance of the face of thepaper or glass from the rear surface of the eyepiece lenswith a scale graduated in millimeters or decimals orfractions of an inch (fig 2-55). Mark the opposite edgesof the exit pupil on the paper or glass and measure itsdiameter in millimeters or decimals or fractions of aninch. The position and diameter of the exit pupil can bemeasured exactly by means of a Ramsden dynameter(para 5-24).9-4. Magnifying Power.

a. Definition. The magnifying power of an opticalinstrument is the ratio of the heights of the retinal imagesproduced in the observer’s eye with and without theinstrument.

b. Calculation. The magnifying power of an opticalinstrument depends upon the relation between the focallength of the objective and the focal length of the lensesof the eyepiece, the latter being considered as a singlelens. The magnifying power of a telescope equals to thefocal length of the objective divided by the focal length ofthe eyepiece. For example, if the objective has anequivalent focal length of 8 inches and the eyepiece hasan equivalent focal length of 1 inch, the instrument willhave a magnification of 8 power.

c. Measurement. The magnifying power of aninstrument can be calculated when the equivalent focallengths of the objective and the eyepiece are

known. It can be determined by measuring the focallengths of the elements. The magnifying power can beapproximately determined by this method: Mark anumber of connected squares of equal size in a line on adistant white wall. Observe the squares with the righteye looking through the instrument and with the left eyelooking at them naturally. The right eye will receive amagnified impression of one or two of the squares; theleft eye will receive a natural impression of a number ofthem. The two impressions will be fused by the braininto a single impression of a large square with a numberof smaller ones crossing it in a line. If four naturalsquares are seen inside the magnified square, theinstrument has a magnification of approximately 4diameters or 4 power.9-5. Measurement of Field of View.

a. The field of view (FOV) of an instrument may beexpressed in either angular or linear measure. Theangular measure is more commonly given and isexpressed in degrees and minutes. The linear measureis expressed in meters at ranges of 100 meters or 1,000meters.

b. To determine angular FOV, place the telescopein a fixture or mount which is provided with an azimuthscale, a micrometer, and levels. Level the instrumentand direct the telescope so that a convenient verticalobject is at the left edge of the FOV. Read the scale andmicrometer. Traverse the instrument until the object is atthe right edge of the FOV. Again read the scale andmicrometer. The difference between the two readings isthe angular measure of the true FOV (para 5-25). If thescale and micrometer are graduated in mils, convert todegrees and minutes.

c. To measure the linear FOV, place two stakes ata specified distance from the instrument; one at eachedge of the observed FOV. Measure the actual distancebetween the two stakes. The linear FOV may bedetermined mathematically by the formula: 2 x tangentof one-half the angular FOV x the range of the stakes.9-6. Determination of Spherical Aberration. Cut amask of black paper that will cover the objective. Cut acircle one-half the diameter of the objective out of thecenter of this mask. Apply the mask to the objective.Sharply focus a distant object on the

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crossline of the instrument. Now, remove the mask andplace the cutout circle on the center of the objective.Examine the image of the object. Any blurring of theimage may be attributed to unfocused rays in the outerzones of the objective. Adjust whatever focusing devicesthere are available to sharply focus the image. Theamount of movement required is an indication of theamount of spherical aberration present.

9-7. Determination of Curvature of Image.Sharply focus the instrument on a point in the center ofthe image and note whether or not the points at the edgeof the image are clear and well defined. If the edges ofthe image are hazy, continue focusing the instrumentuntil the edges of the image are sharp and distinct. Theamount of focusing required is an indication of thecurvature of image.

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CHAPTER 10

MEASUREMENT SYSTEMS EMPLOYED IN OPTICS

10-1. Candlepower and Foot Candles.a. The intensity of illumination on a surface

depends upon the brightness of the light source and thedistance of the surface from the light source. Thebrightness of any light source is measured in units ofcandlepower. One candlepower is the rate at which astandard candle emits light. A standard candle wasinitially a sperm whale oil candle, 7/8 inch in diameter,which burned at the uniform rate of 120 grains per hour.Current standards of candlepower are electric lampswhich may be obtained from the Bureau of Standards.

b. The amount of illumination on any surfacedepends upon the distance of the surface from the lightsource. A candle 1 foot away from a screen will shedmore intense light on a given area of the screen than itwould if it were 3 feet from the screen. In the latter case,it would illuminate a larger area with less intensity. Theintensity of illumination is measured in foot candles. Afoot candle is the amount of light intensity received on asurface 1 foot away from a light source of 1candlepower.

c. The intensity of illumination or the quantity oflight which is received per unit surface varies inverselyas the square of its distance from the source.

Candlepowerfoot candles =

(distance)2

d. The amount of light falling upon a surface ismeasured in lumens. A lumen is the amount of lightfalling on an area of 1 square foot at 1 foot distance froma standard candle (a above). Therefore, total lumensdivided by area equals foot candles of illumination.10-2. Diopters.

a. General. The lens diopter or simply diopter isthe unit of measure of the refractive power of a lens orlens system. The prism diopter is the unit of measure ofthe refracting power of a prism. Both are based on themetric system of measurement.

b. Lens Diopter. A lens with a focal length of 1meter is internationally recognized to have the power of 1diopter. The power of a converging lens is positive andthat of a diverging lens is negative.

The power of other lenses than those of 1-meter focallength is the reciprocal of the focal length in meters andvaries inversely as the focal length. This means that aconverging lens with a focal length of 20 centimeters or1/5 meter has a power of +5 diopters, while a diverginglens with a focal length of 50 centimeters or 1/2 meterhas a power of -2 diopters. The lens with the shortestfocal length has the greatest plus or minus power indiopters.

c. Prism Diopter. A prism with a power of 1 prismdiopter will deviate light by 1 centimeter at a distance of 1meter from the prism. Thus, the prism diopterdesignation of a refracting prism is the measurement ofthe amount it can bend light.10-3. Degree System.

a. The degree system is a means of measuringand designating angles of arcs. The degree is 1/360 partof a circle or the value of the angle formed by dividing aright angle into 90 equal parts. Each degree is dividedinto 60 parts called seconds.

b. In the use of fire-control equipment, eitherdegrees or mils (para 10-4) may be used to designateangles of elevation and azimuth. One degree equals17.78 mils. One mil equals 0.0560 or 3 minutes 22 1/2seconds. Refer to the conversion chart (fig 10-1) for theapproximate relative values of given angles of elevationand azimuth in degrees and mils.10-4. Mil System.

a. The artillery mil system is a means of angularmeasurement that lends itself to simple mentalarithmetic. It provides accuracy within the limitsdemanded by the military forces with distinct advantagesof simplicity and convenience not afforded by any othermethod of angular measurement. It is based on anarbitrary unit of measurement known as the mil. The milis exactly 1/6400 of a complete circle.

b. The mil is very nearly the angle between twolines which will enclose a distance of 1 meter at a rangeof 1,000 meters (fig 10-1). Exact computation of thedistance enclosed at 1,000 meters by 1 mil gives theresult of 0.982 meters. Therefore in assuming that 1 milencloses 1 meter at 1,000 meters, an error of 0.018meter or 1.8 cm is introduced. This error is negligible forall practical purposes in the use of fire-controlinstruments.

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Figure 10-1. Artillery mil-degree conversion chart.

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c. A circle of 1,000 meters radius would have acircumference of 6,283 meters. A 1-meter portion of thecircumference would be the 1/6283 fractional part of a circle.By choosing the mil as the 1/6400 fractional part of a circle,sufficient accuracy is retained for all practical purposes whilethe number can be handled easily by mental arithmetic.

d. The mil provides an extremely small unit of angularmeasurement that is easily adaptable to the small anglesencountered by the artilleryman. For example, if an object hasan angular width or height of 1 mil, it is 1 meter wide or high at1,000 meters; 2 meters wide or high at 2,000 meters; and soon. In angular height, a man is approximately 2

mils tall at 1,000 meters or 1 mil tall at 2,000 meters.

NOTE’The Navy mil and the French infantry mil areexactly 1 meter at a range of 1,000 meters.There are 6283.1853 Navy or Frenchinfantry mils in a complete circle.

e. For quick approximate conversion from artillery milsto degrees, or vice versa, refer to figure 10-1. On this chartlocate the number of degrees or mils to be converted. Directlyacross the black line of arc is its equivalent.

f. For more exact conversion from artillery mils todegrees, minutes, and seconds, use the following listing:

Artillery mils Degrees Minutes Seconds

1....................................................................................... 0 3 22.52....................................................................................... 0 6 453....................................................................................... 0 10 7.54....................................................................................... 0 13 305....................................................................................... 0 16 52.56....................................................................................... 0 20 157....................................................................................... 0 23 37.58....................................................................................... 0 27 09....................................................................................... 0 30 22.5

10....................................................................................... 0 33 4520....................................................................................... 1 7 3030....................................................................................... 1 41 1540....................................................................................... 2 15 050....................................................................................... 2 48 4560....................................................................................... 3 22 3070....................................................................................... 3 56 1580....................................................................................... 4 30 090....................................................................................... 5 3 45

100 ....................................................................................... 5 37 30200 ....................................................................................... 11 15 0300 ....................................................................................... 16 52 30400 ....................................................................................... 22 30 0500 ....................................................................................... 28 7 30600 ....................................................................................... 33 45 0700 ....................................................................................... 39 22 30800 ....................................................................................... 45 0 0900 ....................................................................................... 50 37 301,000..................................................................................... 56 15 02,000..................................................................................... 112 30 03,000..................................................................................... 168 45 04,000..................................................................................... 225 0 05,000..................................................................................... 281 15 06,000..................................................................................... 337 30 06,400..................................................................................... 360 0 0For example, to convert 2,569 mils:

2,000 mils = 1120 30’ 0"500 mils = 280 7’ 30"

60 mils = 30 22’ 30"9 mile = 0° 30' 22.5"

1430 89' 82.5"2,569 mile = 1440 30' 22.5"

g. For more exact conversion from degrees andminutes to artillery mils, use this conversion table:

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Degree Minutes Artillery mils

0 1.....................................................................................300 2.....................................................................................590 3.....................................................................................890 4..................................................................................1.190 5..................................................................................1.480 6..................................................................................1.780 7..................................................................................2.070 8..................................................................................2.370 9..................................................................................2.760 10..................................................................................2.960 02..................................................................................2.960 20..................................................................................5.930 30..................................................................................8.890 50................................................................................14.821 0................................................................................17.782 0................................................................................35.563 0................................................................................53.334 0................................................................................71.115 0................................................................................88.896 0..............................................................................106.677 0..............................................................................124.448 0..............................................................................142.229 0..............................................................................160.00

10 0..............................................................................177.7820 0..............................................................................355.5630 0..............................................................................533.3340 0..............................................................................711.1150 0..............................................................................888.8960 0...........................................................................1,066.6770 0...........................................................................1,244.4480 0...........................................................................1,422.2290 0...........................................................................1,600.00

100 0...........................................................................1,777.78200 0...........................................................................3,555.56300 0...........................................................................5,333.33360 0...........................................................................6,400.00

For example, to convert 78°43’:70° 0' = 1,244.448° 0' = 142.220° 40' = 11.850° 3' = .89

78° 43' =1,399.40 mils

10-5. Metric System.a. The meter, which is the basis of the metric

system, was intended to be and is very nearly one ten-millionth of the distance from the equator to the polemeasured at sea level on the meridian of the earth. Themeter is now defined as the distance between two lineson a certain platinum-iridium bar kept in Paris, when thebar is at 0° C. The value of the meter in wave length oflight is known with very great accuracy. The value forthe wave length of red cadmium radiation underspecified standard conditions is 0.00064384696 mm. All

units of linear measurement of the metric system aremultiples or fractional parts of the meter in the units of10. It is a system based on decimals which providesease of conversion from one to another of the variousunits of the system. Units of measurement are providedfrom extremely small to the very large, inasmuch as theyinclude the physical units of measure of the X-ray Unit,micron, meter, and kilometer.

b. One meter is equal to 39.37 inches. Following isa listing of metric units with their equivalents in inches,yards, and miles:

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= 1 millimeter = 0.03937 inch10 millimeters= = 1 centimeter = 0.3937 inch10 centimeters = 1 decimeter = 3.937 inches10 decimeters = 1 meter = 1.0936 yards10 meters = 1 dekameter = 10.936 yards10 dekameters = 1 hectometer = 10.936 yards10 hectometers = 1 kilometer = 0.6214 mile

c. One inch equals 25.4 millimeters. One footequals 30.48 centimeters. One yard equals 0.914 meter.One mile equals 1.609 kilometers. For quickapproximate conversion from inches to metric

system units, or vice versa, refer to the conversion table(fig 10-2). On this table, locate the measurement to beconverted. Directly across the black line is its equivalent.

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Figure 10-2. Metric unit-inch conversion table.

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d. For more exact conversion and for conversionof large units, use the following factors to convert:

From To MultiplyMillimeters ..................................Inches......................................................Millimeter by 0.03937Inches.........................................Millimeters ............................................... Inches by 25.4Meters ........................................Inches......................................................Meters by 39.37Meters ........................................Yards.......................................................Meters by 1,0936Inches.........................................Meters ..................................................... Inches by 0.0254Yards ..........................................Meters .....................................................Yards by 0.9144Kilometers ..................................Miles........................................................Kilometers by 0.6214Miles ...........................................Kilometers ...............................................Miles by 1.609

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APPENDIX A

REFERENCES

A-1. Publication Indexes. The following indexes should be consulted frequently for latest changes or revisions ofreferences given in this appendix and for new publications relating to materiel covered in this manual:Index of Army Motion Pictures, Film Strips, Slides, and Phono-Recordings.

DA Pam 108-1Military Publications:

Index of Administrative Publications........................................................................................... DA Pam 310-1Index of Blank Forms .................................................................................................................DA Pam 310-2Index of Graphic Training Aids and Devices ..............................................................................DA Pam 310-5Index of Supply Manuals; Ordnance Corps................................................................................DA Pam 310-6Index of Technical Manuals, Technical Bulletins (Type 7, 8,......................................................DA Pam 310-4

and 9), Lubrication Orders, and Modification Work Orders.Index of Doctrinal, Training, and Organizational Publications....................................................DA Pam 310-3

A-2. Other Publications.Elements of Optics and Optical Instruments ......................................................................................ST 9-2601-1Military Symbols ......................................................................................................................FM 21-30Military Terms, Abbreviations, and Symbols: Authorized Abbreviations ............................................AR 320-50and Brevity Codes.Dictionary of United States Army Terms ............................................................................................AR 320-5Military Training FM 21-5Techniques of Military Instruction.......................................................................................................FM 21-6

A-3. Textbooks.A College Textbook of Physics by Arthur L. Kimbal-Henry Holt and Co., New York - 1939. ThePrinciples of Optics by Arthur C. Hardy and Fred H. Perrin - McGraw-Hill Book Co., Inc., NewYork and London - 1932.

A-1

APPENDIX B

GLOSSARY

B-1. General. This glossary contains definitions of anumber of specialized terms found in the study of optics,in the use of sighting and fire-control equipment, andwith reference to the eyes and vision. It includesdefinitions of most of the more unusual words used inthis manual. The majority of the italicized words in thismanual will be found in this glossary. When in doubt asto the meaning of a word, refer to this glossary.B-2. Glossary.

Abbe prism - Two right-angle prisms joined to formone prism (fig 4-16). Used as component of one form ofthe Abbe prism system (figs 4-10 and 4-21).

Abbe prism erecting system - A prism erectingsystem in which there are four reflections to erect theimage. It is composed of two double right-angle prismswhich bend the path of light 360�, displacing but notdeviating the path of light (fig 4-21).Aberration - Any defect of a lens or optical

system which causes the image to be imperfect. Seeastigmatism, chromatic aberration, coma, curvature ofimage, distortion, and spherical aberration.

Absorption - The act of process by which an objector substance "takes up" or "soaks up" all the colorscontained in a beam of white light except those colorswhich it reflects or transmits. Because of absorption,objects appear to have different colors; white objectshave little absorption; other objects having varyingpowers of absorption appear to have different colors anddifferent shades of brightness. For example, an objectwhich appears red has absorbed all the color of thespectrum except the color red. This is known asselective absorption or selective reflection. A piece ofruby glass placed between the eye and a source of whitelight allows only red light to pass through because itabsorbs all other colors. This is called selectiveabsorption or selective transmission (fig B-1).

Figure B-1. Absorption-Selective transmission.

Accommodation - Automatic adjustment of thecrystalline lens of the human eye for seeing objects atdifferent distances. The process whereby the crystallinelens is adjusted to focus successive images of objectslocated at various distances from the eye. See limits ofaccommodation.

Achromatic - Without color.Achromatic lens - A lens consisting of two or more

elements, usually made of crown and flint

glass, which has been corrected for chromaticaberration. See compound lens.

Acuity - Keenness; sharpness. See stereo-acuityand visual acuity.

Adapter - A tube, ring, or specially formed partwhich serves to fit or connect one part with another, ormount an element of smaller diameter into a part oflarger diameter.

Aiming circle - An instrument for measuring

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horizontal and vertical angles and for generaltopographic work.

Aiming point - The point on which the gunner sightswhen aiming the gun. This point is not necessarily thetarget itself.

Ametropia - Any abnormal condition of the seeingpower of the eyes, such as farsightedness(hypermetropia), nearsightedness (myopia), orastigmatism. An ametropic eye is one which does notform distinct images of objects on its retina.

Amici prism - Also called "roof prism" and "roof-angle prism." A form of roof prism designed by G. B.Amici, consisting of a roof edge formed upon the longreflecting face of a right-angle prism

(fig 4-11). Used as an erecting system in elbow andpanoramic telescopes. It erects the image and bendsthe line of sight through a 90° angle.

Anastigmat - A compound lens corrected forastigmatism. Angle The amount of rotation of a linearound the point of its intersection with another,necessary to bring it into coincidence with the secondline.

Angle of azimuth - An angle measured clockwise ina horizontal plane usually from north (fig B-2). The northused may be True North, Magnetic North, or Y-North(Grid North).

Figure B-2. Angle of azimuth.

Angle of convergence - Angle formed by the lines ofsight of both eyes in focusing on any point, line, corner,surface, or part of an object Also referred to asconvergence angle (figs 3-8 and 3-9).

Angle of deviation - The angle through which a rayof light is bent by a refracting surface; the angle betweenthe extension of the path of an

incident ray and the refracted ray (fig 2-24).Angle of elevation - The angle between the line of

site (line from gun to target) and line of elevation (axis ofbore when gun is in firing position) (fig B-3). Thequadrant angle of elevation is the angle between thehorizontal and the line of elevation. Angle of elevationplus angle of site equals quadrant angle of elevation.

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Figure B-3. Angle of elevation, angle of site, and quadrant angle.

Angle of incidence - The angle between the normal(perpendicular) to a reflecting or refracting surface andthe incident ray (figs 2-13 and 2-24).

Angle of reflection - The angle between the normalto a reflecting surface and the reflected ray (fig 2-13).The incident ray, reflected ray, and normal all lie in thesame plane.

Angle of refraction - The angle between the normalto the refracting surface and the refracted ray (fig 2-24).The incident ray, refracted ray, and normal all lie in thesame plane.

Angle of site - The vertical angle between thehorizontal and the line of site (line from gun to

target) (fig B-3).Angstrom unit - A unit of measure equaling one ten-

millionth part of a millimeter.Angular - Composed of or measured by angles.Antiglare diaphragm - See diaphragm.Aperture - An opening or hole through which light or

matter may pass. In an optical system, it is equal to thediameter of the largest entering beam of light which cantravel completely through the system. This may or maynot be equal to the aperture of the objective (fig B-4).

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Figure B-4. Apertures.

Aperture of objective - The diameter of that part ofthe objective which is not covered by the mounting.

Aperture stop - The diaphragm which limits the sizeof the aperture. See diaphragm.

Aplanatic lens - A lens which has been corrected forspherical aberration, coma, and chromatic aberration.

Apochromatic lens - A lens, usually consisting ofthree components of different kinds of glass (two crownglass elements, one flint glass element), which has beencorrected for chromatic aberration with respect to threeselected colors or wavelengths of light.

Apparent field of view - The angular size of the fieldof view of an optical instrument, as seen through theinstrument by the eye (fig 2-51). See field of view.

Aqueous humor - The transparent liquid which

is contained between the cornea and the crystalline lensof the eye (fig 3-2).

Arc - A part of the circumference of a circle.Artillery - mil See mil.Asthenopia - Weakness or rapid fatigue resulting

from use of the eyes indicated by headache or pain inthe eyes. Often referred to as weak sight or eyestrain.

Astigmatisma. An aberration or defect of a lens which

causes a point of the object off the axis to be imaged asa short line or pair of short lines. When two lines areformed, each is at a different distance from the lens andis at an angle to the other, and the lens has two points ofprincipal focus (B, fig B-5). A sharp image cannot besecured at either focal point and the best results areobtained at a point between the two focal points in aplane known as the circle of least confusion (fig 2-64).

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Figure B-5. Astigmatism

b. A defect of the human eye causing straight linesin a certain direction to appear blurred and distortedwhile lines in another direction may be well defined. Thisdefect results when rays from a point are not brought toa single focal point on the image on the retina but a shortline is formed. It is caused by unsymmetrical surfaces ofthe cornea and the crystalline lens of the eye (A, fig B-5).

Astigmatizer - A cylindrical lens which may berotated into the line of sight of a range finder to cause theeffect of astigmatism, to stretch a ray of light from asingle point to form a line.

Axis (plural, axes) - A straight line passing through abody and indicating its center. See axis of bore, principalaxis, and optical axis.

Axis of bore - The center line or straight lineB-5

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through the center of the bore of a gun (fig B-6). The lineof sight of the sighting instrument of a

gun is adjusted parallel to the axis of bore.

Figure B-6. Axis of bore and boresights-boresighting.

Axis of lens - See principal axis.Azimuth - An angle measured clockwise in a

horizontal plane from a known reference point frequentlyone of the norths: True North, Magnetic North, or Y-North (Grid North). (See angle of azimuth). Also usedas an adjective to indicate reference to movement in anhorizontal plane as azimuth mechanism or azimuthinstrument. To traverse the carriage of a weapon inazimuth means to rotate the weapon from side to side ina horizontal plane.

Azimuth instrument - A telescopic instrument usedfor measuring horizontal angles.

Azimuth mechanism - Any mechanical meansprovided for turning an instrument in azimuth (inhorizontal plane). It may contain a worm and wormwheelto give accurate, smooth movement.

Backlash - A condition wherein a gear, forming partof a gear train, may be moved without moving the nextsucceeding or preceding gear. It

is due to space between the teeth of the meshing gears.Balance - See orthophoria.Balsam - See Canada balsam.Base length - In range finders, the actual optical

length of the instrument. It is approximately equal to thedistance between the centers of the end windows. It isthe base of the range triangle by means of which therange is computed (fig 8-23).

BC periscope (Battery Commander’s Periscope) - Abinocular telescope used for observing artillery fire.

Beam - With reference to light, a shaft or column oflight; a bundle or rays. It may consist of parallel,converging, or diverging rays.

Binocular - Pertaining to vision with both eyes. Also,a term applied to instruments consisting of twotelescopes utilizing both eyes of the observer.

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Blind spot - The part of retina, inner coat of eyeball,not sensitive to light where the optic nerve enters.

Bore sight - A sighting device consisting of a breechelement and a muzzle element which, inserted in a gun,is used to determine the axis of the bore and thealinement of other sighting equipment with the axis of thebore. A bore sight may consist of a metal disk with apeephole for the breech and a pair of crosslines for themuzzle (fig B-6).

Boresight - To adjust the line of sight of the sightinginstrument of a gun to the axis of the bore (fig B-6).

Brightness of image - A term used to denote theamount of light transmitted by an optical system to givedefinition to the image seen by the observer.

Burnishing - The process of turning a thin edge ofmetal over the edge of a lens to hold it in place in its cell(fig B-7). This eliminates the use of a retaining ring.Burnished optics are usually procured as assemblies,inasmuch as it is difficult to replace their componentparts.

Figure B-7. Burnishing

Case I pointing (or firing) - Direct pointing, laying, orfire; gun pointing in which direction and elevation are setwith sight of telescope pointed at the target.

Case II pointing (or firing) - Combined direct andindirect pointing, laying, or fire; gun pointing in whichdirection is set with a sight or telescope pointed at thetarget and the elevation with an elevation quadrant,range quadrant, or range disk.

Case III pointing (or firing) - Indirect pointing, laying,or fire; gun pointing in which direction is set with anazimuth circle or with a sight or telescope pointed at anaiming point other than the target; the elevation is setwith an elevation quadrant, range quadrant, or rangedisk.

Cataract - A diseased condition of the human eye inwhich the cornea or crystalline lens becomes opaqueresulting in blindness.

Cell - A tubular mounting used to hold a lens in itsproper position. The lens may be held in the cell byburnishing or by a retaining ring.

Center of curvature - The center of the sphere ofwhich the surface of a lens or mirror forms a part. Eachcurved surface of a lens has a center of curvature(fig B-8). The curved surfaces may be convex orconcave.

Figure B-8. Centers of curvature

Calcite - See Iceland spar.Canada balsam - A clear, practically colorless sap

of fir, solidifying to a transparent resin used in cementingoptical elements together particularly the components ofa compound lens. It is used because it hasapproximately the same index of refraction as glass.

Candlepower - A unit of measure of the brightnessor power of any light source. One candlepower is therate at which a standard candle, initially made of spermwhale oil, 7/8 inch in diameter, burning at the rate of 120grains per hour, emits light. Current standards ofcandlepower are electric lamps.

Centric Pencil - Oblique pencil or cone of light whichpasses through the center of a lens at a considerableangle to the principal axis.

Choroid - The opaque middle coat or tunic of thehuman eye. It is a deep purple layer made up of bloodvessels; it supplies nourishment to the eye tissues andshuts out external light (fig 3-2).

Chromatic aberration - An aberration (deviation fromnormal) of a lens which causes

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different colors of wavelengths of light to be focused atdifferent distances from the lens resulting in coloredfringes around the borders of objects seen through thelens (fig 2-60).

Ciliary body or muscle - The muscle in the eyeballwhich is capable of increasing or decreasing thecurvature of the crystalline lens to decrease or increasethe focal length of the lens system of the eye. Used inaccommodation (focusing) of the eye (fig 3-4).

Circle of least confusion - A focal plane between thetwo focal points of a lens affected by astigmatism wherethe most clearly defined image can be obtained. Seeastigmatism.

Coated optics - Optical elements which have beencoated with a thin chemical film, such as magnesiumfluoride, to reduce reflection and thereby increase lighttransmission.

Coincidence - Occupying the same place; agreeingas to position; corresponding. In the coincidence rangefinder, two optical systems are focused into a singleeyepiece assembly. When the range knob is rotated, theleft and right fields of view are superimposed andcoincidence of the two images of the selected target isestablished.

Coincidence prism - A compound prism, consistingof a system of small prisms cemented together, used asan ocular prism assembly in a coincidence range finderto bring the image from the two objectives to a singleeyepiece for viewing.

Coincidence range finder - A self-containeddistance measuring device operating on the principle oftriangulation. Images, observed from points of knowndistance, are alined to determine the range. Seecoincidence.

Collective lens - A convex or positive lens used inan optical system to collect the field rays and bend themto the next optical element. It prevents loss of rim raylight. Sometimes used to denote a convergent or convexlens.

Collimating telescope - A telescope with an outercylindrical surface that is concentric with its optical axis.It contains a reticle, usually straight crosslines. It is usedto aline the axis of the optical elements to themechanical axis of the instrument (collimation) in othertelescopes, to focus eyepieces, and to set diopter scalesat infinity focus.

Collimation - The process of alining the axis of theoptical elements to the mechanical axis of an instrument.

Collimator - A sighting instrument occupying aposition between the open sight and the telescopicinstrument (fig 8-27). It has an eyelens, and reticle ofcross or vertical line pattern.

Color blindness - An eye condition of not beingcapable of proper discrimination of colors. In the mostcommon type, dichromatism, red-green blindness, thesetwo colors appear gray.

Coma - An aberration of a lens which causesoblique pencils of light from a point on the object to beimaged as a comet-shaped blur instead of a point. Thisaberration is caused by unequal refraction through thedifferent parts of the lens for rays coming from a pointwhich lies a distance off the axis (fig 2-65).

Compass North (Magnetic North) - The directionindicated by the north-seeking end of the needle of amagnetic compass. This direction is different usuallyfrom either True North or Y-North (Grid North),depending upon one’s position upon the earth’s surface.

Compensator - See measuring wedge.Compound lens - A lens composed of two or more

separate pieces of glass. These component pieces orelements may or may not be cemented together. Acommon form of compound lens is a two elementobjective; one element being a converging lens of crownglass and the other being a diverging lens of flint glass.The surfaces of the two elements are ground to eliminateaberrations which would be present in a single lens (fig4-3).

Concave - Hollowed to form a shallow cavity androunded like the inside of a sphere.

Concave lens - See diverging lens.Concave-convex lens - A lens with one concave

surface and one convex surface.Concentric - Having the same center. Circles

differing in radius but inscribed from a single centerpoint.

Cone - One of the two types of light-sensitiveelements or visual cells in the retina of the eye whichpermit sight. The cones are associated with daylightvision and give clear sharp vision for the seeing of smalldetails and the distinguishing of color. The other type ofvisual cell is termed the rod (fig 3-5).

Conjugate focal points (conjugate foci) - Those pairsof points on the principal axis of a concave mirror or aconvergent lens so located that light emitted from eitherpoint will be focused at the other as shown in figure B-9where D and D- are the conjugate focal lengths of a lens.Shifting the source of light along the axis will cause ashifting of the second corresponding conjugate focalpoint. Likewise related points in object and image arelocated optically so that for every point on the objectthere is a corresponding point in the image.

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Figure B-9. Conjugate focal points (conjugate foci).

Constant deviation - A certain amount of deviationgiven to the line of sight or optical axis of one of theinternal telescopic systems of a range finder in order thatthe correction wedge may adjust the system by supplyinga plus, neutral, or minus correction, as required.

Converge - As applied to the eyes or binocularoptical instruments, to direct them so that the two lines ofsight meet at a common focus and form an angle. Todirect light rays or lines to a small area.

Convergence - See angle of convergence.

Convergent lens - See converging lens.Converging lens - Also known as convergent lens,

positive lens, convex lens, collective lens. A lens that willconverge light. One surface of a converging lens may beconvexly spherical and the other plane (planoconvex),both may be convex (double convex) or one surface maybe convex and the other concave (convexo-concavemeniscus converging). A converging lens is alwaysthicker at the center than at the edge (fig B-10).

Figure B-10. Converging lenses.

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Converging meniscus lens - See meniscusconverging lens.

Convex - Rounded and bulging outwardly as theouter surface of a sphere.

Convex lens - See converging lens.Convexo-concave lens - A lens with one convex

and one concave surface.Cornea - The transparent, slightly bulging front

surface of the eyeball through which all light enters(fig 3-2). It serves as one of the lenses of the eye.

Corrected lens - A compound lens, the varioussurfaces of which have been so designed with respect toeach other that the lens is reasonably free from one ormore aberrations.

Correction wedge - In range finders a rotatingwedge-shaped element used to precisely divert the lineof sight to correct errors in the optical system caused bytemperature variations. In range finders, serves tosupply a measuring factor correction for the constantdeviation given one of the telescopes. The measuringfactor correction is read on a scale.

Correction window - End correction window. Theseare optical wedges of very small angles. They admitlight, seal out dirt and moisture, and are so mounted thatthey may be rotated to compensate for the accumulatederrors in the entire system. Two are used as endwindows on some range finders.

Critical angle - That angle at which light, about topass from a medium of greater optical density, isrefracted along the surface of the denser medium (fig 2-34). When this angle is exceeded, the light is reflectedback into the denser medium. The critical angle varieswith the index of refraction of the substance or medium.

Cross-eye - See strabismus.Crown glass - One of the two principal types of

optical glass; the other type is flint glass. Crown glass isharder than flint glass, has a lower index of refraction,and lower dispersion. See compound lens.

Crystalline lens - The flexible inner lens of the eyewhich provides accommodation for sharply focusing nearand distant objects (fig 3-2).

Curvature of field - See curvature of image.Curvature of image - An aberration of a lens which

causes an image to be focused in a curved plane insteadof a flat one (A, fig 2-68).

Cylindrical lens - A lens having for one of itssurfaces a segment of a cylinder. Lenses ground in thismanner are used in spectacles to correct astigmatism ofthe eyes and in some of the coincidence range finders tocause astigmatism by stretching points of light into linesof light.

Dark-adapted - A term applied to the adjustment ofthe visual cells of the retina of the eye for better visionunder conditions of poor lighting. It

applies to the process whereby the rods of the retinatake over the major portion of the act of seeing.

Definition - Sharpness of focus: distinctness ofimage.

Deflection - A turning aside from a straight course.The horizontal clockwise angle from the line of fire to theline of sight to the aiming point. A small horizontal(traverse) angle by which a gun is aimed slightly awayfrom its target to allow for factors such as wind and drift.See drift.

Degree - A unit of measurement equal to the anglebetween radii subtended by 1/360 of a circle.

Depth perception - The ability to see in depth orthree dimensions. In addition to stereoscopic vision, lightand shade, perspective, convergence of the visual axesof the two eyes, one object or part concealing another,atmospheric haze, and apparent size are factors whichaid depth perception.

Deviation - Turning aside from a course; deflection;difference. In optics, the bending of light from its path.In gunfire, the distance from a point or center of impactto the center of a target.

Dialyte - A type of compound lens in which the innersurfaces of the two elements are ground to differentcurvatures to correct for aberrations and the dissimilarfaces cannot be cemented together (fig 4-3). Gaussobjective (para 4-3b).

Diaphragm - A flanged or plain ring with a limitedaperture placed in an optical system at any of severalpoints to cut off marginal rays of light not essential to thefield of view. Diaphragms are used as: field stops, toreduce aberrations; aperture stops, to limit the apertureor light-gathering power of the instrument; and antiglarediaphragms, to eliminate reflections from the sides of thetube and consequent glare in the field of view. Lens cellsor the sides of the tube may act as diaphragms. Therays eliminated by diaphragms are those which wouldcause aberrations or glare and ghost images byreflection inside the instrument.

Dichromatism - See color blindness.Diffusion - The scattering of light by reflection or

transmission. Diffuse reflection and transmission resultwhen light strikes an irregular surface such as a frostedwindow or the surface of a frosted light bulb. When lightis diffused, no definite image is formed.

Diopter - A unit of optical measurement whichexpresses the refractive power of a lens or prism. In alens or lens system, it is equivalent to the reciprocal ofthe focal length in meters. For example, if a lens has afocal length of 25 centimeters, this figure converted intometers would equal 1/4 meter. In as much as thereciprocal of 1/4 equals 1 divided by 1 which equals 4,such a lens would be said to have

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a power of 4 diopters. The lens with the shorter focallength has the greater power in diopters. See prismdiopter.

Diopter movement - A term applied to adjustment ofthe eyepiece of an instrument to provide accommodationfor eyesight variations of individual observers.

Diopter scale - A scale usually found on thefocusing nut of the eyepiece of an optical instrument. Itmeasures the change in refracting power of the eyepiecein diopters to introduce a correction to compensate forthe-nearsightedness or farsightedness of the individualobserver. It permits presetting of the instrument if theobserver knows his diopter correction.: Dioptometer -Precision instrument used in determining the dioptersetting of another instrument.

Dioplopia - See double vision.Discernible difference of convergence angles - The

differences in the angles of view from the two eyes toobjects or parts of objects. These differences, althoughsmall, permit persons with normal, two-eyed vision todistinguish which of two objects is farther away, makingstereoscopic vision possible without resorting to the otherfactors which aid depth perception.

Dispersion - In optics, the separation of a beam ofwhite light into its component colors as in passingthrough a prism (fig 2-51). See spectrum.

Dispersive lens - See diverging lens.Distortion - The aberration of a lens or lens system

which causes objects to appear misshapen or deformedwhen seen through the lens or lens system. This defectis known as "barrel-shaped" distortion when the center ofthe field of view is enlarged with respect to the edgesand "hourglass" or "pincushion" distortion when theedges are enlarged with respect to the center (fig 2-67).Wavy lines are sometimes produced by improperlyshaped or polished windows, prisms, and mirrors. Whencaused by a lens, it is due to varying magnification ofdifferent parts of the image.

Diverge - In a lens, to deviate the light outward froma common center in different directions. As applied tothe eyes, the action of the pupils being directed outward(Walleye vision) or in not being brought to a commonfocus.

Divergent lens - See diverging lens.Diverging lens - Also known as divergent lens,

negative lens, concave lens, dispersive lens. A lenswhich causes parallel light rays to spread out. Onesurface of a diverging lens may be concavely sphericaland the other plane (planoconcave), both may beconcave (double concave) or one surface may beconcave and the other convex (concavo-convexmeniscus diverging). The diverging lens is alwaysthicker at the edge than at the center (fig B-11).

Figure B-11. Diverging lenses.

Diverging meniscus lens - See meniscus diverginglens.

Double concave lens - A lens with two concavesurfaces (fig 4-1).

Double convex lens - A lens with two convexsurfaces (fig 4-1).

Double right-angle Abbe prism - See Abbe prism.Double vision -

a. A malfunction of a binocular instrumentcausing two images to be seen separately instead ofbeing fused. It is caused by the optical axes of the

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two telescopes not being parallel or not converging to apoint. In minor cases, the eyes will adjust themselves tocompensate for the error of the instrument until theimages are superimposed and only one object is seen.This may cause eyestrain, eye fatigue, and headaches.

b. An eye disorder, known as diplopia, whichresults in double vision of a single object. It is usuallycaused by one eye failing to converge or diverge inunison with the other eye.

Doublet - A compound lens consisting of twoelements with inner surfaces curved identically and

cemented together (fig 4-3).Dove prism - Used as a rotating prism. A form of

prism designed by H. W. Dove. It is used to invert theimage in one plane without deviating or displacing theaxis of the rays of light. Used as the rotating prism in theconventional type of optical system of panoramictelescopes (figs 4-13 and 4-22).

Drift - The amount by which a projectile will deviatehorizontally from its proper path, due to rotation causedby the rifling in the bore of the gun, and reaction of the air(fig B-12).

Figure B-12. Drift.

Dynameter- A Ramsden dynameter is a smalleyepiece or magnifier equipped with a reticle scale andused in the precise measurement of the exit pupil andeye distance (eye relief) of other optical instruments.

Eccentric mounting - A type of lens mountingconsisting of eccentric rings which may be rotated to shiftthe axis of the lens to a prescribed position (fig 4-36).

Eccentric ray - One of the rim rays which passthrough a lens remote from its center.

Effective aperture of objective - See aperture ofobjective.

Elbow telescope - A refracting optical instrument forviewing objects in which the line of sight is bent 90degrees by means of a prism.

Electromagnetic spectrum - A chart or graphshowing the relation of all known electromagnetic waveforms classified by wavelength. The visible lightspectrum occupies an extremely minute portion of theelectromagnetic spectrum (fig 2-3).

Elementary lens equation - A law giving thequantitative relation between the distance of the object,the image, and the principal focus of the lens (para 2-24.e).

Emergent ray - In optics, the term applied to a ray oflight leaving an optically dense medium as

contrasted with the entering or incident ray (fig 2-24).Emmetropia - Normal refractive condition of the

eyes. A normal eye is termed emmetropic.End correction window - See correction window.End play - Movement of a shaft along its axis. A

type of lost motion common to worm and wormwheelassemblies. The error lies in looseness in the bearingsat the ends of the shaft or in the ball cap and socket.The result is that the worm can be rotated a smallamount without causing rotation of the wormwheel.

Entrance pupil - The clear aperture of the objective.Equilibrium, equipoise - Muscular balance of the

eyes.Erect - To change an image from an inverted to a

normal position. To both revert and invert.Erect image - The image produced by an optical

system which is seen with its upper part up. The normalerect image appears as it is seen by/the normal eye.The reverted erect image is seen with the right side onthe left.

Erect type - The image produced by one type ofcoincidence range finder in which the image appears

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normal erect when in coincidence except for thepresence of the halving line.

Erecting system - Lenses or prisms, the function ofwhich is to erect the image, that is, to bring the imageupright after it has been inverted by the

objective. An erecting system may consist of one ormore lenses (figs B-13 and 4-20), each of which is calledan erector, or of one or more prisms (figs 4-21 and 4-11).

Figure B-13. Lens erecting system.

Erector - One of the lenses of a lens erectingsystem (fig B-13).

Esophoria - A condition of the eyes in which thelines of sight tend to turn inward.

Etching - The marking of a surface by acid, acidfumes, gas, or a tool. A process extensively used in themanufacture of reticles.

Exit pupil - The diameter of the bundle of lightleaving an optical system. The small circle or disk oflight which is seen by looking at the eyepiece of aninstrument directed at an illuminated area (fig 2-53). Itsdiameter is equal to the entrance pupil divided by themagnification of the instrument.

Exophoria - A condition of the eyes in which thelines of sight tend to turn outwardly.

Extra-foveal vision - Vision in parts of the retinaother than fovea.

Eye distance or eye relief - The distance from therear surface of the eyelens to the plane of the exit pupil(fig 2-55). The center of rotation of the observer’s eyeshould be situated in this plane. The center of rotation ofthe eye is about 6 millimeters behind the vertex of thecornea.

Eyeguard - See eyeshield.Eyelens - The lens of an eyepiece which is nearest

to the eye (fig B-14). See eyepiece. Various types oflenses are used for this purpose.

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Figure B-14. Eyelens and fields lens of eyepiece.

Eyepiece - An optical system used to form a virtual,erect, enlarged image of the real image formed by theobjective (fig B-14). The optical system of the eyepieceusually consists of two lenses, an eyelens and a fieldlens, but the eyepiece may contain another lens (figs 4-5,4-6 and 4-7).

Eye relief - See eye distance.Eyeshield or eyeguard - A shield of rubber plastic,

or metal to protect the eyes of the observer from straylight and wind and to maintain proper eye distance.

Far point - The farthest point of clear vision of theunaccommodated eye. For the normal eye, the far pointis infinity. See infinity.

Farsightedness - See hypermetropia.Field glass - A type of compact binocular.Field lens - One of the leneses of an eyepiece. It is

the lens which is nearest the image upon which theeyepiece is focused. It serves to shorten the eye reliefand adds to brightness of the edge of the field (fig B-14).

Field of view - The open or visible space or anglecommanded by the eye. It

is the maximum angle of view that can be seen at onetime. In an instrument, the true field of view is the actualangle of view of the instrument, the maximum anglesubtended at the objective by any objects which can beviewed simultaneously. The apparent field of view is thesize of the field of view angle as it appears to the eye; itis approximately equal to the magnifying power of theinstrument times the angle of the true field of view(fig 2-52).

Field stop - See diaphragm.Filters or ray filters - Colored glass disks with plane,

parallel surfaces placed in the path of light through theoptical system of an instrument to reduce glare and lightintensity and likewise in observing tracer fire anddetecting outlines of camouflaged objects. They areprovided as separate elements or as integral devicesmounted so they may be placed in or out of position asdesired (fig 4-27).

Finite - Having limits, as opposed to infinite, orwithout limits.

Fire control - The determination and regulation ofthe direction of gunfire. More specifically, it refers

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to the observation and calculations necessary fordetermining the correct aiming of a weapon system.

Fixed focus - The term applied to instruments whichare not provided with means for focusing. Suchinstruments, generally, have a wide range ofaccommodation which permits them to be used by themajority of observers. Collimator is an example.

Flint glass - One of the two principal types of opticalglass, the other being crown glass. Flint glass is softerthan crown glass, has a higher index of refraction, andhigher dispersion. See compound lens.

Fluorescence - Phenomenon whereby light of onewavelength is absorbed by a material, and thenreemitted as light of a different wavelength. Fluorescentlight sources require generally external white light tocause them to become luminous and glow with a coloredhue. They cease to become luminous when the escitingagent is removed.

Focal length - The distance from the principal focus(focus of parallel rays of light) to the surface of a mirroror the optical center of a lens (figs 2-44, 2-45, 2-46 andB-15)

Figure B-15. Focal length of lens and mirror.

Focal plane - A plane through the focal pointperpendicular to the principal axis of a lens or mirror (fig2-44).

Focal point - The point to which rays of lightconverge or from which they diverge when they havebeen acted upon by a lens or mirror (figs 2-46 and 2-62).A lens has many focal points, depending upon thedistance of the object from the lens.

Focus -a .To adjust the eyepiece of a telescope so that

the image is clearly seen by the eye or to adjust the

lens of a camera so that a sharp, distinct image is seenon the ground glass.

b. The process of adjusting the distances betweenoptical elements to obtain the desired optical effect.

c. Same as focal point.Focusing nut - A threaded nut in the center of which

the eyepiece of a telescope is attached to permit theeyepiece to be moved in or out to accommodate theinstrument to eyesight variations. It

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usually carries a diopter scale to permit presetting of theinstrument (fig 4-38).

Focusing sleeve - A knurled sleeve which is rotatedto shift the positions of the erectors with relation to theobjective and eyepiece to focus the instrument or tochange its magnification.

Foot-candles - A unit of measurement of the intensityof illumination. A foot-candle is the amount of lightintensity received on a surface 1 foot away from a lightsource of one candle power, such as a standard candleof sperm whale oil, 7/8 inch in diameter, burning at therate of 120 grains per hour.

Fovea or fovea - centralis A tiny area of cones in thecenter of the macula or the yellow spot of the retina ofthe eye. It is the area of the eye responsible for theclearest vision. It is about 0.25 millimeter in diameter,and contains cones only.

Freedom from defects - Optical glass which ishomogeneous and free from bubbles or striae (localizedvariations in the index of refraction).

Freedom from distortion - An image that is a truereproduction of the object.

Frequency - In light or other wave motion, thenumber of crests of waves that pass a fixed point in agiven unit of time, usually 1 second.

Front surface mirror - An optical mirror on which thereflecting coating is applied to the front surface of themirror insead of to the back.

Fuse - In stereoscopic vision, the action of seeing asone image the images seen by the two eyes.

Fusion - The mental blending of the right and left eyeimages into a single, clear image by stereoscopic action.

Galilean telescope - One of the first telescopemodels, utilizing a positive objective lens and divergingeyelens (figs 5-13 and 5-14).

Gauss - objective See dialyte.Gear train - Two or more gears meshed together so

that rotation of one causes rotation of the others.Geometrical center - As applied to lenses, the

physical center of the lens as determined bymeasurement; sometimes referred to as the mechanicalcenter to distinguish it from the optical center.

Ghost image - A hazy second image caused byreflection sometimes seen in observing through atelescope.

Gimbal joint - A mechanism which permits rotationaround two perpendicular axes or for suspending anobject so that it will remain vertical or level when itssupport is tipped.

Grid declination - The angle between True Northand Y-North.

Grid North - See Y-North.Gunner’s quadrant - An instrument with a

graduated arc, used in range adjustment. It measuresthe angle of elevation of the gun.

Halving line - The line which divides the two halfimages in a coincidence type range finder. The twohalves of the images produced by the two objectives ofthe instrument must be brought to a point where theymatch or coincide above and below the halving line.

Hard coat - The term applied to the coating ofmagnesium fluoride on coated optics to differentiatebetween other softer coatings which are less durable.

Height of image adjuster - A glass plate with planesurfaces which is tipped one way or the other in the lineof sight in one of the internal telescopes of a rangefinder. It deviates the light slightly upward or downwardto adjust the image of one eyepiece to the same heightas the image visible in the other eyepiece.

Heterophoria or muscular imbalance - Anydisturbance of the power, strength, or nervous system ofthe eye muscles which does not permit both eyes tofunction together in a normal way. It may occur in any ofa number of different forms such as esophoria in whichthe lines of sight tend to turn inwardly, esophoria in whichthey tend to turn outwardly, and hyperphoria in which theline of sight of one eye tends to be above that of theother.

Homogeneity - Being uniform throughout.Homogeneous - Uniform throughout; composed of

similar parts.Horizontal travel - Rotation of an instrument (or the

line of sight of an optical instrument) in a horizontalplane; traverse; movement in azimuth.

Hypermetropia - An eye defect in which the imagetends to be focused beyond the retina with the result thatthe image is blurred and indistinct, farsightedness. It iscaused by the refracting surfaces of the eye being tooslightly curved, by the eye being of insufficient depth, orby the decline in the power of accommodation in viewingnearby objects. It is corrected by a converging lens (fig3-11).

Hyperopia - See hypermetropia.Hyperphoria - A condition of the eyes in which the

line of sight of one eye tends to be above that of theother.Hyperphoric displacement - Action of the eyes necessaryto obtain fusion in observing through a stereoscopic orbinocular instrument when the image seen by one eye isabove that seen by the other eye.

Iceland splar, calcite - A mineral that has the facultyof polarizing light.

Illuminated - Lighted by another source than

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itself, as opposed to a luminous body, which is a lightsource.

Image - A reproduction or picture of an objectproduced by light rays. An image-forming opticalelement produces an image by collecting a beam of lightdiverging from an object point and trans forming it into abeam which converges toward, or

diverges from, another point. If the beam converges to apoint, a real image is produced (fig 2-47); if the beamdiverges, a virtual image is produced at its apparentsource (fig 2-47). A real image can be thrown upon ascreen (fig 2-47); a virtual image exists only in the mindof the observer.

Figure B-16. Virtual image produced by diverging lens .

Figure B-17. Virtual image produced by diverging lens .

Image plane - The plane in which the image lies or isformed. It is perpendicular to the axis of the lens and isat the focal point. A real image formed by a converginglens would be visible upon a screen placed in this plane(fig 2-46).

Imbalance - See Heterophoria.Incandescence - The state of a body in which its

temperature is so high that it gives off light.Examples: the sun or the filament of an electric lamp.

Incidence - The act of falling upon, as light upon asurface.

Incident ray - A ray of light which falls upon or strikesthe surface of an object such as a lens or mirror. It issaid to be incident to the surface (figs 2-13 and 2-24), ascontrasted with the ray leaving an optically densemedium or emergent ray.

Index (plural, indices)a. An arrow or mark against which a

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calibration, graduation, or scale is set to indicate theextent to which a mechanism is adjusted; something thatpoints out.

b. The ratio of one dimension or quantity toanother.

Index of refraction - A number applied to atransparent substance which denotes how much fasterlight will travel in a vacuum than through that particularsubstance. It is equal to the ratio between the speed oflight ’in a vacuum to the speed of light in a substance. Itdetermines the relation between the angle of incidence(i) and the angle of refraction (r) when light passes fromone medium to another (figs 2-24 and B-18). The indexbetween two media is called the relative index, while theindex when the first medium is a vacuum is called theabsolute index of the second medium. The relative indexis the ratio of the sine of the angle of incidence to thesine of the angle of refraction, or the speed of light in thefirst medium to the speed of light in the second medium.If the first medium is air, this relation is expressed as -= sin i = Speed of light in airn sin r Speed of light in other medium(Snell’s law)where n equals very nearly the absolute index ofrefraction, i equals the angle of incidence, and r equalsthe angle of refraction.

Figure B-18. Index of refraction

NOTEThe index of refraction expressed in tables is theabsolute index, that is, vacuum to substance at acertain temperature, with light of a certainwavelength. Examples: vacuum, 1.00; air,1,000293; water 1.333; ordinary crown glass,1.517. Since the index of air is very close to thatof vacuum, the two are often usedinterchangeably as being practically the same.

Infinite - Without limit, as opposed to finite, meaningwith limits.Infinity - Distance having no end or limits. Greater thanany assignable distance or quantity. In optics, the termis used to denote a distance sufficiently great so that lightrays coming from that distance are practically parallel toone another. Infinity is indicated by the symbol ∞

Infrared radiation - Electromagnetic radiation whosewavelength lies just beyond the red end of the visiblespectrum, and the beginning of the region that can bedetected by microwave radio techniques.

Interpupillary - Between the pupils of the eyes.Interpupillary adjustment - The adjustment of the

distance between the eyepieces of a binocularinstrument to correspond with the distance between thepupils of the eyes of an individual.

Interpupillary distance - The distance between thecenters of the pupils of the eyes. It is generally stated inmillimeters. It varies with the individual.

Inversion - Turned over; upside down so that topbecomes bottom and vice versa. It is the effectproduced by a horizontal mirror in reflecting an image.

Invert-type Image - The type of image observed incertain coincidence range finders. When in coincidence,the upper half image appears to be the mirroredreflection of the lower half image.

Inverted image - Turned over; upside down. Animage, the top of which appears to be the bottom of theobject and vice versa. Usually refers to the effect of amirror, a prism, or lens upon the image.Inversion is the effect of turning upside down. A planemirror held under an object will produce an invertedimage. The normal inverted image is seen upside down,with the right side on the right. The reverted invertedimage is seen upside down with the right side on the left.

Iris - The colored part of the eye about the pupil.Depending upon the intensity of light, the iris causes thepupil to dilate or contract.

Jack screw - A screw threaded through one part andpressing against another. A jack (portable machine) inwhich a screw is used for lifting or for exerting pressure.

Laser - Acronym for light amplification by stimulatedemission of radiation.

Law of reflection - The angle of reflection is equal tothe angle of incidence; the incident ray, reflected ray, andnormal all lie in the same plane (fig 2-13).

Law of refraction - When light is passing from anoptically lighter medium to an optically denser medium,its path is deviated, toward the normal; when passinginto an optically less dense medium,

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its path is deviated away from the normal (fig 2-24).Theamount of deviation is determined by means of theequation for the index of refraction for the two mediainvolved. See index of refraction.

Law of relative size of object and image - The size ofthe image is to the size of the object as the imagedistance from the lens or mirror is to the object distance.

Law of reversibility - If the direction of light isreversed, it will travel in the opposite direction over thesame path despite the number of times it is reflected orrefracted.

Lens (plural, lenses) - A transparent object, usually apiece of optical glass, having two polished surfaces of’which at least one is curved, usually with a sphericalcurvature. It is shaped so that rays of light, on passingthrough it, are made to converge or diverge. The factthat lenses either converge or diverge rays provides onemeans of classification, the type of curvature providesanother (fig 4-1), the corrections made in the lensprovides still another. The term lens may be employedto mean a compound lens which can consist of two ormore elements. See achromatic lens, anastigmat,aplanatic lens, apochromatic lens, astigmatizer,collective lens, compound lens, concavo-convex lens,converging

lens, convexo-concave lens, corrected lens, cylindricallens, dialyte, diverging lens, double-concave lens,double-convex lens, doublet erector eyelens, field lens,meniscus-converging lens, meniscus diverging lens,minus lens, objective, orthoscopic lens, plano-concavelens, planoconvex lens, plus lens, and triplet.

Lens cell - A number of lenses mounted as a unit ina tubular mounting frame.

Lens diopter - See diopter.Lens equation, elementary - The equation giving the

relation between the distances from the optical center ofthe lens of the object (Do), the image (Di), and theprincipal focus (F). These values are related as

1 plus 1 equals 1Do Di F

Lens erecting system - See erecting system.Lens retaining ring - See retaining ring.Lens system - Two or more lenses arranged to work inconjunction with one another (fig B-19). When in properalinement, the principal axes will be coincident and thesystem can be referred to as a "centered lens system."

Figure B-19. Optical system of erecting telescope .

Lenses of the eye - The refractive elements of theeye. See cornea and crystalline lens.

Light-adapter - A term applied to the adjustment ofthe visual cells of the retina of the-eye for the clearestand most distinct ’sight under good lighting conditions. Itapplies to the process whereby the cones of the retinatake over the major portion of the act of seeing.

Light ray - The term applied to the radii of waves oflight to indicate the direction of travel of the light. Lightrays are indicated by arrows and lines (fig 2-5).

Limiting angle of resolution - The angle subtended bytwo points which are just far enough apart to permit themto be distinguished as separate points. It is commonlyused as a measure of resolving power.

Limits of accommodation The distances of thenearest and farthest points which can be focused clearlyby the eyes of an individual. Usually varies from 4 to 5inches to infinity. See accommodation.

Line of elevation In artillery, the line of the axis of thebore of a gun when it is in firing position (fig B-3).

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Line of position - See line of site.Line of sight - Line of vision; optical axis of a

telescope or other observation instrument. Straight lineconnecting the observer with the aiming point; the linealong which the sights are set in a gun, parallel to theaxis of the bore (fig B-6).

Line of site A straight line extending from a gun orposition-finding instrument to a point, especially a target(fig B-3). Usually called line of position.

Figure B-20. Linear field

Lost motion - Motion of a mechanical part which isnot transmitted to connected or related parts. It is thecumulative result of backlash and end play.

Lumen - The amount of light falling on an area of 1square foot, at 1 foot distance from a standard candle.See standard candle.

Luminescence - Giving off light at a temperaturebelow that of incandescence. The radiating of "cold" lightas seen from fireflies or luminous paint.

Macula or yellow spot - A small area of the retina ofthe eye which is highly sensitive to light and which is thezone of the clearest and most distinct vision except for amuch smaller area at its center known as the fovea orfovea centralis. The macula is about 2 millimeters indiameter, and contains principally cones and only a fewrods (fig 3-2).

Magnesium fluoride - See hard coat.Magnetic azimuth - Azimuth measured from

Magnetic North.Magnetic declination - The angle between True

North and Magnetic North

Magnetic-grid declination - The angle betweenMagnetic North and Grid North (Y-North).

Magnetic needle - A magnetized needle used in acompass. When freely suspended, it will assume aposition parallel to the earth’s magnetic lines of forcewhich connect the magnetic poles.

Magnetic North - The direction of the earth’s northmagnetic pole as indicated by the north-seeking end of amagnetic needle.

Magnification - The increase in the apparent size ofan object produced by an optical element or instrument.

Magnifying power - The ability of a lens, mirror, oroptical system to make an object appear larger (fig B-21). If an optical element or optical system makes anobject appear twice as high and twice as wide, theelement or system is said to have a magnification of 2-power. The power of an optical instrument is thediameter of the entrance pupil divided by the diameter ofthe exit pupil, the focal length of the objective divided bythe focal length of the eyepiece, or the apparent field ofview divided by the true field of view.

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Figure B-21. Magnifying power .

Marginal rays - Rays of light near the edge of a lens.Maser - Acronym for microwave amplification by

stimulated emission of radiation.Measuring wedge - An optical element employed in a

range finder to deviate or displace light entering thevariable angle end of the instrument. One type consistsof a single, perpendicular wedge which is moved inalinement with the path of light to shift or displace thelight (fig 4-18). The other type consists of one or twopairs of circular wedges, each pair being capable ofrotating equally and simultaneously in opposite directionsabout the axis of the path of light to produce variabledeviation (fig 4-19). The wedges of both types arecompound elements corrected for color.

Medium (plural, media) - Any substance or space.Meniscus - A lens the surfaces of which are curved

in the same direction.Meniscus converging lens - A lens the surfaces of

which are curved in the same direction; one surface isconvex, the other is concave. The convex surface has’the greater curvature or power (fig 4-1). Spectaclelenses of this type are used to correct farsightedness,hypermetropia, or hyperopia (fig 3-11).

Meniscus diverging lens - A lens the surfaces ofwhich are curved in the same direction; one surface isconvex, the other is concave. The concave surface hasthe greater curvature or power (fig 4-1). Spectaclelenses of this type are used to correct nearsightedness,myopia (fig 3-10).

Micrometer - A mechanism for measuring smallangles or dimensions. On optical instruments it is the"fine" scale.

Micrometer scale - An auxiliary scale used to readsmall angles or dimensions.

Micromicron - A unit of measure equalling one-millionth part of a micron.

Micromillimeter or millimicron - Terms applied to unitof measure equal to one-millionth part of a millimeter orone-thousandth part of a micron.

Microna - A unit of measure equaling one--millionthpart of a meter or one-thousandth part of a millimeter.

Mil (Artillery Mil) - A unit of angular measurementused in Army military calculations. It is 1/6400th of acomplete circle or very nearly the angle between twolines which will enclose a distance of 1 meter at a rangeof 1,000 meters or 2 meters at a range of 2,000 meters(fig 10-2). 17.78 mils is approximately equal to 1 degree.

NOTEThe Navy mil and the French infantry milenclose exactly 1 meter at a range of 1,000meters. There are 6283.1853 Navy or Frenchinfantry mils in a complete circle.

Minus lens - A diverging lens. A lens with negativefocal length (focal point toward the object).

Minute - A unit of measurement of an angle equal toone-sixtieth of a degree.

Mirror - A smooth, highly polished surface forreflecting light. It may be plane or curved. Usually a thincoating of silver or aluminum on glass constitutes theactual reflecting surface. When this surface is applied tothe front face of the glass, the mirror is termed a frontsurface mirror.

Monochromatic - Composed of one color.Monocular - Pertaining to vision with one eye.

A term applied to optical instruments requiring the use ofonly one eye

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Mount - A device for attaching or supporting anoptical instrument. It may be may not be provided withmeans for elevating and traversing the instrument,calibrations to indicate degree of movement, and meansand gages for leveling. This term is applied also tomeans of securing an optical element in an instrument.

Muscular imbalance - See heterophoria.

Myopia - An eye defect which does not permitdistant objects to be seen clearly, nearsightedness.It is the result of eyes that do not conform to standardmeasurements, having too much optical depth or toogreat curvature of surfaces. The image tends to befocused before it reaches the retina and is indistinct. It iscorrected by a diverging lens (fig B-22).

Figure B-22. Nearsightedness (myopia) .

Navy mil - See mil.Near point - Closest point of clear vision with the

unaided eye.Nearsightedness - See myopia.Negative lens - See diverging lens.Newton’s rings - A series of concentric colored or

bright and dark circles seen when positive and negative(convergent and divergent) surfaces of nearly the samecurvature are pressed together. It is caused byinterference of light rays. It is used to test lenses. Themore nearly the surfaces are matched, the greater thedistance between the rings and the larger and moreregular the circles (fig 2-70).

Night blindness - A condition of the eyes (andpossibly the health) of a person which does not permithim to see as well at night or with poor illumination as theaverage person.

Night glasses - An optical instrument for use at nighthaving an exit pupil at least 7 millimeters.

Normal - An imaginary line forming right angles witha surface or other lines, the perpendicular. It is used asa basis and reference line for determining angles ofincidence, reflection, and refraction. In figure B-23, thenormal is perpendicular or at right angles to all lines lyingin the plane and passing through point 0.

Figure B-23. Normal and oblique .

Normal erect - See erect image.Normal inverted - See inverted image.North - See Magnetic North, True North, Y-North

(Grid North).Object - The principal thing imaged by the optical

system. It may be part of large object, an entire objectplus its immediate surroundings, or a number of objects.It is a general term used for the sake of convenience.

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Object distance - The distance of an object from theeyes or from an optical system.

Objective - The lens in an optical system whichreceives light from the field of view and forms the firstimage. It is the lens farthest from the eye and nearestthe object viewed.

Objective cap - A protective device, usually made ofleather, which is placed over the objective end of theinstrument when the instrument is not in ’use (fig 4-37).

Objective prism - A right-angle prism employed insome types of instruments to bend light 90° before itenters the objective system (fig 4-22).Oblique Slanting; neither at right angles nor parallel (figB-23)

Ocular -1. A term sometimes applied to the eyepiece.2. Pertaining to, connected with, or used for the

eyes or eyesight.Ocular prism - The prism employed in a range finder

to bend the line of sight through the instrument into theeyepiece.

Off-carriage instruments - Those instruments used inartillery which are not mounted on the carriage of theweapon such as the aiming circle, BC periscope, andrange finder.

Offset - See prism offset.Oldsightedness - See presbyopia.On-carriage instruments - Those instruments used in

artillery which are mounted directly on the carriage of theweapon such as panoramic and elbow telescopes andtheir associated telescope mounts.

Ophthalmic - Pertaining to the human eye.Optic nerve - Nerve connecting the eye and the optic

centers of the brain.Optical - Pertaining to vision and the phenomenon of

light.Optical axis - The line formed by the coinciding

principal axes of a series of optical elements comprisingan optical system; in other words, an imaginary linepassing through the optical centers of the system.

Optical center - Point in a lens on a line connectingall focal points, peculiar in that it is a point through whichthe rays of light pass and emerge parallel to their path ofincidence (fig 2-45). It is in the geometric center of thethickest part of a converging (convex) lens and of thethinnest part of a diverging (concave) lens.

Optical characteristics - Those properties of anoptical system which it possesses by reason of its opticalor visual nature such as field of view, magnification,brightness or image, and freedom from distortion.

Optical - element A part of an optical instrumentwhich acts upon the light passing through the instrumentsuch as a lens, prism, or mirror.

Optical glass - Glass carefully manufactured toobtain purity, transparency, homogeneity, and correctoptical properties. See crown glass and flint glass.

Optical properties - In optical glass, those propertieswhich pertain to the effect of the glass upon light such asindex of refraction, dispersion, homogeneity, andfreedom from defects.

Optical surface - A reflecting or refracting surface.Optical system - A number of lenses, or lenses and

prisms, or mirrors, so arranged as to refract, or refractand reflect light, to perform some definite optical function(fig B-19).

Optics - That branch of physical science which isconcerned with the nature and properties of light, relatedinstruments, and vision.

Orbit - The socket of the human eye.Orient - In fire control, to find the proper bearing; to

fix the position of a gun, with reference to apredetermined target point.

Orientation - Determination of the horizontal andvertical location of points and the establishment oforienting lines, adjustment of the azimuth circle of aweapon or instrument to read required azimuths, andadjustment of the vertical quadrant angle of elevation(angle of site plus angle or elevation) to required values.

Orthophoria - A term used with reference to the eyesindicating that both eyes are equally and properlycontrolled by eye muscles with the result that, with theeyes at rest, they focus and converge upon a distantobject.

Orthoscopic eyepiece - An eyepiece which has beencorrected for distortion, particularly that type of eyepiecehaving a triple element cemented field lens and aplanoconvex or meniscus converging eyelens(paras 2-24i, 4-4k, and B, fig 4-7). See eyepiece.

Othoscopic lens - A lens which has been correctedfor distortion, giving an image in correct or normalproportions, with a flat field of view (para 2-24i).

Panoramic telescope - A telescope so designed thatthe image remains erect and the position of the \fs20eyepiece is unchanged as the line of sight is pointed inany horizontal direction.

Parabolic mirror - A concave mirror which has theform of a special geometrical surface; a paraboloid ofrevolution. Light rays emanating from a source locatedat the focal point of the paraboloid are reflected asparallel rays. Most searchlights are of this design. Alllight rays which are parallel and strike the mirror directlyfrom the front are reflected towards the focal point(fig 2-21).

Parallactic angle - In range finding, the angle at thetarget subtended by the base length. The

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baseline of the range finder forms the base of a triangleand its apex is the target (fig. 8-28). Corresponds to theangle of convergence in stereoscopic vision (fig. 3-8 and3-9).

Parallax - Any apparent displacement of an objectdue to the observer’s change of position such as in A,figure 2-72, where the observer will see either the polenumber 4, or pole 7 positioned opposite the mountain, ifhe changes his position. This illusion of a relativedisplacement of the poles with respect to the mountain isa deception, inasmuch as neither the poles nor themountain actually move. Optically, parallax in atelescope with a reticle is any apparent movement of thereticle in relation to distant objects in the field of viewcaused by movement of the head of the observer. Thiscondition exists when the image in the telescope lies inone focal plane and the reticle lies in another(B, fig 2-72).

Paraxial pencil - A narrow group of light rays alongthe axis.

Paraxial ray - Any ray parallel to the axial ray in apencil of light. A ray in the immediate neighborhood ofthe optical axis of a lens or mirror or close to its center.

Patina - A film. Surface mellowing or softening. Afilm or oxide formed on copper and bronze.

Pechan or Z-Rotating prism - Used as a rotatingprism. It is used to invert the image in one plane withoutdeviating or displacing the axis of the rays of light. Usedas the rotating prism in the conventional type of opticalsystem of panoramic telescopes (B, fig 4-13).

Pencil of light - A narrow group of light rays comingfrom a point source or converging toward a point. Apinhole opening produces a pencil of rays.

Pentaprism - A five-sided prism used to bend lightthrough a constant angle, usually 90 degrees, withoutproducing inversion or reversion if reflection takes placein the vertical or horizontal plane (fig.4-14).

Peripheral - On the circumference. Near theboundary or edge of the field of an optical system; theouter fringe.

Periscope - An optical instrument designed todisplace the line of sight in a vertical direction usuallyupwards. Used to permit observation over the top of afortification, a barricade, or out of a tank (fig 8-19).

Persistence of vision - The mental effect of an

impression registered upon the retina and the optic nerveremaining for about 1/15 second after the stimulus hasbeen removed.

Perspective - Appearance in terms of distance.Example, railroad tracks which appear to converge asthey are seen receding in the distance. Appearance inrelief, in three dimensions.

Phosphorescence - The glowing or giving off of lightwithout sensible heat; the property to shine in the darkdue to emission of light caused by chemical reaction, asfireflies.

Photon - A quantum (elemental unit) of radiantenergy.

Plane - A surface which has no curvature; a perfectlyflat surface.

Planoconcave lens - A lens with one surface plane;the other concave.

Planoconvex lens - A lens with one surface plane;the other convex.

Plumbline - A vertical line. When used for adjustinginstruments, it is a weight (plumb bob) hanging on astring.

Plus lens- A converging (convex) lens.Point of fixation - The object on which the optical

instrument is focused; an object on which the observer’seye is concentrated.

Point of principal focus - The point to which parallelrays of light converge or from which they diverge whenthey have been acted upon by a lens or mirror (figs 2-19,2-44, and 2-46). A lens has two points of principal focus,one on each side of the lens. A lens has many focalpoints depending upon the distance of the object fromthe lens (fig 2-47).

Polarized light - In optics, the light which vibrates inone direction only (in a single plane).Plane of polarization is the plane in which the polarizedlight vibrates. Light is polarized by natural materials(Iceland spar) or a manufactured product known as"Polaroid" or when being reflected, at a glancing anglefrom a smooth surface.

Polarizing filter - A light ray filter which polarizes thelight passing through an instrument.

Porro prism - One of two identical prisms (fig B-24)used in the Porro prism erecting system (fig 4-9). It is aright angle prism with the corners rounded to minimizebreakage and simplify assembly. See porro prismerecting system (fig 4-9).

B-24

Figure B-24. Porro prism .

Porro prisms erecting system A prism erectingsystem, designed by M. Porro, in which there are fourreflections to completely erect the image. Two Porroprisms are employed (fig 4-9). The line of sight is bentthrough 3600, is displaced, but is not deviated.

Positive lens - See converging lens.Power -

1. In a prism or a lens, power is a measure ofability to bend or refract light. It is usually measured indiopters. See diopter and prism diopter.

2. In a telescope, power is the number oftimes the instrument megnifies the object viewed. Forexample, with a 6-power instrument, an object 600 yardsaway is enlarged six times or it appears as it would to thenaked eye if it were at a distance of only 100 yards.

Presbyopia - A defect of vision due to advancingage, "oldsightedness". The crystalline lens no longer iscapable of accommodating for nearby objects and theseobjects cannot be seen distinctly. It is corrected by theuse of convex lenses.

Principal axis - A straight line about which an opticalelement or system is symmetrical. A straight lineconnecting the centers of curvature of the two refractingsurfaces of a double convex lens. In a mechanicalsense, a line joining the centers of the two surfaces of alens as it is placed in the mount.

Principal focal plane - Plane perpendicular to theaxis through the point of principal focus (figs 2-44 and2-46).

Principal focal point - See point of principal focus.Principal focus - See point of principal focus.Prism - A transparent body with at least two polished

plane faces inclined toward each other from which light isreflected or through which light is refracted. When lightis refracted by a prism, it is deviated or bent toward thethicker part of the prism (fig B-25). See Abbe prismerecting system, Amici prism, coincidence prism, Doveprism, objective prism, Pechan prism, pentaprism, Porroprism, rhomboidal prism, right-angle prism, and triplemirror.

Figure B-25. Prism .

Prism diopter A unit of measure of the refractingpower of a prism. One diopter is the power of a prism todeviate a ray of light by one centimeter at a distance ofone meter from the prism.

Prism erecting system - See erecting system.Prism offset - The term applied to certain telescopes

having a characteristic offset due to the

mounting of the prism erecting system in the body of theinstrument.

Prismatic - Pertaining to a prism or the effectsproduced by prisms.

Probable error - An allowance based on averageerrors made by a large group of range finder

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operators. It will be exceeded as frequently as it is not.Protractor An instrument for measurng or laying off

angles. It consists of a graduated arc or

semicircle, sometimes containing a radial arm (fig B-26).The graduations can be Artillery mils, Navy mils, ordegrees. See mil.

Figure B-26. Protractor .

Pupil - The dark center of the eye. It is the aperturethrough which light enters the eye.

Quadrant -1. A quarter of a circle: a sector, arc, or angle

of 90 degrees.2. An instrument for measuring or setting

vertical angles such as a gunner’s quadrant or a rangequadrant.

Quadrant angle of elevation - The angle between thehorizontal and the line of elevation or axis or bore(fig. B-3). See angle of elevation and angle of site.

Quanta or Quantum - See photon.Radian - An angle included within an arc equal in

length to the radius of a circle. It is equal to 57°, 17minutes, and 44.8 seconds.

Radiant energy - Energy that is radiated or thrown offin all directions by its source. Light is a form of radiantenergy. Energy, by definition, is capacity for performingwork.

Ramsden dynameter - See dynameter.Range Distance. - The horizontal distance from a

gun to its target; the distance from an observer to adesignated object. Range usually is measured inmeters.

Range finder - An optical instrument used todetermine the distance of an object or target by

triangulation which is performed mechanically andoptically.

Range quadrant - An instrument used to set range ormeasure vertical angles of elevation in laying a gun.

Ray - See light ray.Ray filter - See filters.Real image - See image.Rectilinear - In a straight line. When applied to a

lens, it indicates that images of straight lines produced bythe lens are not distorted.

Rectilinear propagation - Straight line travel; refers tothe fact that light travels in a straight line when travelingthrough a medium of constant optical density.

Reduction - The decrease in the apparent size of anobject produced by an optical element or instrument.

Reflected ray - The ray of light leaving a reflectingsurface representing the path of light after reflection (fig2-13).

Reflection - Light striking a surface and returning or"bouncing back" into the medium whence it came.Regular reflection (figs 2-13 and 2-15) from a planepolished surface, such as a mirror, will return the majorportion of the light in a definite direction lying in the planeof the incident ray and

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the normal. See angle of reflection. Regular reflectionwill form a sharp image. Diffuse reflection (fig 2-16)occurs when the surface is irregular and the reflectedlight diverges from each point as if it were a separatereflecting surface. Diffused rays are scattered, go inmany directions, and will not form a distinct image.

Refracted ray - The ray of light passing through andleaving a refracting surface representing the path of lightafter refraction (fig 2-24).

Refracting power - The power of a lens or lenssystem to converge or diverge light. See diopter.

Refraction - The bending of light which occurs whena ray of light passes obliquely from one medium toanother of different optical density. See angle ofrefraction and angle of deviation.

Refractive index - See index of refraction.Regular reflection - See reflection.Relief - Effect of stereoscopic or three dimensional

vision; solidity or depth; sharpness of outline due tocontrast of the object standing out, from a background.

Resolution - In optics, the ability of a lens system toreproduce an image in its true sense. Forming separateimages of two objects or points very close together.

Resolving power - A measure of the ability of a lensor optical system to form separate images of two

objects or points close together. No lens or opticalsystem can form a perfect image of a point; it will appearas a small disk surrounded by concentric circles. If twopoints are so close together that the disks overlap, thepoints cannot be distinguished separately; they are notresolved. The measure of the resolving power is theangle subtended at the optical center of the lens by twopoints which are just far enough apart to permitresolution into two separate images. This angle istermed the limiting angle of resolution.

Retaining ring - A thin ring threaded on the outsidesurface. It is screwed into a tube, cell, or other bodymember of an optical instrument to retain or hold a lensor other part in fixed position.

Reticle - Marks or patterns placed in the focal planeof the objective of an optical instrument which appear tothe observer to be superimposed upon the field of view(fig B-27). They are used as a reference point forsighting or aiming; to measure angular distance betweentwo points; to determine the center of the field; or toassist in the gaging of distance, determining leads, ormeasurement. The reticle may be a pair of crosslinescomposed of fine wire or may be etched on a glass platewith plane parallel surfaces. If it is etched on glass, theentire piece of glass is referred to as the reticle. (Also,see figs 4-24 and 4-25).

Figure B-27. Reticle patterns superimposed on image of object .

Retina - The light-sensitive inner coat or tunic of theeyeball upon which the image is formed by the lenses ofthe eye. It contains the visual cells called the cones androds (figs 3-2 and 3-5).

Reversion - Turned the opposite way so that

right becomes left and vice versa. It is the effectproduced by a vertical mirror in reflecting an image.

Reverted erect - See erect image.Reverted image - An image, the right side of which

appears to be the left side of the object and vice versa.

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Reverted inverted - See inverted image.Rhomboidal prism - A prism with two pairs of parallel

sides forming right angles and two 45� slanting oroblique parallel ends. It will displace the path of lightentering its ends without changing the direction of light.It does not invert or revert the image (fig 4-12).Rhomboidal prisms may be rotated to divert the lines ofsight to permit interpupillary adjustment of the eyepiecesof a binocular instrument.

Right-angle prism - A type of prism used to turn abeam of light through a right angle (900) (fig 2-36). It willinvert (turn upside down) or revert (turn right or left),according to position of the prism, any light reflected byit. This prism is likewise used to turn a beam of lightthrough an angle of 180� (fig 2-43) when either a normalerect image or inverted, reverted image is produced,depending upon the position of the prism with respect tothe object and the observer.

Figure B-28. Right-angle prism .

Rod - One of the two types of light-sensitiveelements or visual cells in the retina of the eye whichpermit sight. The rods are associated with night visionand sight under other conditions of weak light. They alsodetect motion. For seeing in weak light, they arestimulated by a substance known as visual purple. Theother type of visual cell is termed the cone (fig 3-5).

Roof-angle prism or roof prism - See Amici prism.Rotating prism - See Dove prism and Pechan prism.Rotating wedge - A circular optical wedge mounted

to be rotated in the path of light to divert

the line of sight to a limited degree. See correctionwedge, measuring wedge.

Rouge - A material for polishing optical glass madeprincipally from red oxide of iron. The name is alsoapplied to polishing materials which are not red, such asblack rouge and white rouge.

Sclera - The outer of the three coats or tunics of theeyeball. It is tough, white, and flexible and is the whiteportion of the eye normally seen. The slightly protrudingtransparent portion of the center front of the eye, thecornea, is part of this coat (fig 3-2).

Second - A unit of measurement of an angle equal toone-sixtieth of a minute or 1/3600th part of a degree.

Selective absorption - See absorption.Selective reflection - See absorption.Selective transmission - See absorption.Separator - Also known as spacer. A hollow tubular

part used to separate two lenses at a definite distance(fig 4-35).

Sighting instruments - Devices or instrumentsdesigned to aid in the pointing of a weapon, observationand azimuth or range determination, and looking overobstacles.

Sine - The sine of an angle is the side of a right-angle triangle opposite the angle divided by thehypotenuse (the long side opposite the right angle).

Site Location - See angle of site.Soft coat - A term designating the soft coating

applied to coated optics to differentiate between theharder and more durable coating of magnesium fluorideknown as hard coat.

Spacer- See separator.Spectrum - The band of colors produced by a prism

in dividing white light into its components.The rainbow is an example, dispersion produces thisspectrum. See electromagnetic spectrum.

Spherical aberration - The aberration of a lens whichresults when rays of light which pass through a lens nearits edge are converged to a point nearer the lens thanthose rays passing through near the center (fig. 2-55).The effect is poor definition of the image.

Split field - The field of view as seen when observingthrough a coincidence range finder. It is formed byuniting halves of the images produced by two objectives.The half images are separated by the halving line.

Squint - See strabismus.Stadia scale - Graduations on a reticle which in

conjuction with a rod of definite length can be used tomeasure distances.

Standard candle - Initially a sperm whale oil candle,7/8 inch in diameter, burning at the rate of 120 grains perhour. Current standards of candle power are electriclamps.

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Standard letters - Letters of specific size which areviewed at specified distance (20 feet) to test thekeenness of sight.

Stereoacuity - Keenness or sharpness of sight indiscerning depth or three dimensions.

Stereovision See stereoscopic vision.Stereogram - A card containing two pictures taken

from different angles (corresponding to the spacingbetween the pupils of the eyes), which when viewedthrough a stereoscope present a single image with theillusion of depth or three dimensions.

Stereopsis - See stereoscopic vision.Stereoscope - A binocular instrument used to view

stereograms. See stereogram.Stereoscopic contact - A term applied to the action of

bringing the target into the same apparent distance planeas the central measuring mark (pip) of the reticle in theuse of a stereoscopic range finder.See stereoscopic range finder.

Stereoscopic effect - The sense of depth, relief, orsolidity resulting when an object is viewed by both eyes,due to the fact that each eye views the object from aslightly different angle (fig 3-7).Similar effect produced when viewing a stereogram.See stereogram.

Stereoscopic power - The gain in stereoscopic effectafforded by a magnifying binocular instrument ascompared with the ability of the naked eyes. This powerwill vary with the separation of the objectives and thepower of the instrument.

Stereoscopic range finder - A self-contained distancemeasuring device operating on the principle oftriangulation, Images, observed from points of knowndistance by the eyes, result in a sense of depth, and aremade to appear in the same distance plane as thecentral measuring mark (pip) of the stereo reticle. Seestereoscopic contact.

Stereoscopic vision - Vision in depth or threedimensions due to the spacing of the eyes. This spacingof the eyes permits them to see objects from slightlydifferent angles. An impression of the shape, depth, andposition of an object with relation to other objects isreceived when the brain fuses the two separate picturesseen by both eyes into a single image (fig 3-7).

Strabismus - Affection of the eye in which the lines ofsight cannot be directed to the same object at oncebecause of abnormal contraction, relaxation of, or injuryto one or more of the muscles controlling the movementof the eyeball. A condition of the eyes in which the linesof sight tend to turn inwardly is known as convergentstrabismus, esophoria, crosseye, or squint. A conditionof the eyes in which the lines of sight tend to turnoutward is known as divergent strabismus, exophoria, orwall eye.

Strain - Distortion or fault in optical glass caused byinternal stress or tension and brought

about by improper cooling or annealing duringmanufacture of the glass.

Straie - Localized variations in the index of refractionof a piece of optical glass, usually occurring in streaks. Itis caused by improper mixing of the ingredients duringmanufacture.

Sunshade - A tubular projection provided to protectthe objective from the direct rays of the sun (fig 4-37).

Surface plate - A plate having a very accurate planesurface used for testing other surfaces or to provide atrue surface for accurately locating testing fixtures. It isusually mounted on three adjustable legs so that it canbe accurately leveled in a horizontal plane.

Tangent - A straight line which touches a curve or acurved surface but does not cut in.

Telescope - An optical instrument containing asystem of lenses, usually with magnifying power, whichrenders distant objects more clearly visible by enlargingtheir images on the retina of the eye, or whichsuperimposes a reticle pattern on the image of the objectto assist in aiming or in the gaging of distance (figs 5-6,6-8, 5-9, and 5-13).

Thermoplastic cement - Cement capable of beingrepeatedly softened by increase of temperature andhardened by lowering of temperature, such as resinderived from Canada balsam. It is used because it hasapproximately the same index of refraction as glass.See Canada balsam.

Thermosetting cement - A clear, colorless cementused in cementing optical elements together, particularlythe components of a compound lens. It is used becauseit has approximately the same index of refraction asglass. It changes into a substantially infusible productwhen cured under application of heat.

Toroidal (toric) lens - A lens having for one of itssurfaces a segment of a tore (the surface described by acircle or a curve rotating about a straight line in its ownplane). Used in spectacles to correct astigmatism of theeyes. See astigmatism.

Total internal reflection - Reflection which takesplace within a substance because the angle of indidenceof light striking the boundary surface is in excess of thecritical angle. See critical angle.

Tourmaline - A mineral that has the faculty ofpolarizing light.

Trajectory - The curved path through the airdescribed by a projectile in its flight (fig B-3).

Traverse - Movement in azimuth; rotation in ahorizontal plane. See azimuth.

Triple mirror prism - A prism with four triangularsurfaces arranged like two triangular roofs placed at rightangles to one another. Three of the four surfaces areused as mirrors in reflecting light. This prism has theproperty of deviating,

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through an angle of 1800, any ray of light entering it (fig4-15).

Triplet - A three-element compound lens with innersurfaces curved identically and cemented together(fig 4-3).

True field of view - See field of view.True North - Geographic north; the direction along thegeographical meridian of the geographic north pole froma point on the earth’s surface.

Unit of correction - An arbitrary scale graduation aslaid off on the correction wedge scale of a range finder.

Unit of error (UOE) - Unit of measurementcorresponding to 12 seconds of arc. When checking theaccuracy of an observer’s readings of range, asdetermined by a range finder, it is necessary to havesome unit of measure. It has been determined by testthat the mean error of the normal, well-trained observerunder good conditions of observation should not exceed12 seconds of arc for a series of readings, regardless ofthe range involved. This mean error is taken as thebasis for determining the accuracy of range finderreadings. The unit of error (UOPE) is the unit ofmeasure arbitrarily selected and all errors in rangereadings are converted to this unit during analysis.

Velocity of light - The speed of light. It is numericallyequal to the frequency multiplied by the wavelength. Thespeed of light in vacuum equals 186,330 miles persecond.

Vertex - Highest or lowest, the most or least

projecting point on the axis of a convex or concavemirror or lens.

Vertical travel - Movement of the line of sight of anoptical instrument or gun in a vertical plane.

Virtual base - The actual base or baseline of a rangefinder multiplied by the power of magnification of theinstrument.

Virtual image - The image formed by a diverginglens. It is formed by rays which come from the directionof the image but do not originate in it.It cannot be thrown upon a screen. It exits only in themind of the observer (fig 2-50). Similar image formed bya converging lens if the object is inside one focal length(fig 2-48). Apparent position of reflected point whichappears to be located behind the mirror (figs 2-40 and2-41).

Vision persistence - See persistence of vision.Visual acuity - Acuteness or sharpness of sight.

It is measured by the ability to distinguish letters ofspecified size at a given distance.

Visual purple - A substance surrounding the light-sensitive visual cells in the retina of the eye andstimulating the rods for keener sight under poor lightconditions.

Vitreous humor - A transparent, jelly-like substancewith which the rear and greater portion of the eye is filledand which is part of the refracting mechanism of the eye(fig 3-2).

Walleye - See strabismus.Wave - Vibration; a form of movement by which all

radiant energy of the electromagnetic spectrum isassumed to travel (fig B-29).

Figure B-29. Waves and have fronts .

Wave front - A surface normal (at right angles) to abundle of rays as they proceed from a source. The wavefront passes through those parts of the waves which arein the same phase and go in the same direction. Forparallel rays, the wave front is a

plane; for rays diverging from or converging toward apoint, the wave front is spherical (fig B-29).

Wavelength - Length of a wave measured from anypoint on one wave to the corresponding point, in thesame phase on the next wave; usually measured

B-30

TM 9-258

from crest to crest (fig 2-9). Wavelength determines thenature of the various forms of radiant energy whichcomprise the electromagnetic spectrum; it determinesthe color of light (fig 2-3).

Wedge - An optical element with plane inclinedsurfaces, Usually the faces are inclined toward oneanother at very small angles. Wedges divert light towardtheir thicker portions. They may be circular, oblong, orsquare. See correction wedge and measuring wedge.

White light - Light, such as sunlight and daylight,which is composed of all the different wavelengths of thevisible spectrum.

Window - A piece of glass with plane parallelsurfaces used to admit light into an optical instrument butexclude dirt and moisture. Also see correction window.

Worm - A shaft with a gear in the form of a screwwhich meshes with a worm-wheel. The axes of theworm and worm-wheel are generally-perpendicular toeach other.

Wormwheel - A gear designed to mesh with a worm.X-ray unit - A unit of measure equaling one

ten-millionth part of a micron, or one-ten thousandmillionth part of a millimeter.

Yaw - The angle between the axis of the projectile atany moment and the tangent to the trajectory.

Y-azimuth - The azimuth rading taken from the Y-North direction. See Y-North.

Yellow spot - See macula.Y-North - Grid North. The north direction as

determined by the grid lines on a map. The UnitedStates is laid out in seven grid zones. The central gridline of each of these zones points to True North. Theother grid lines being parallel to the central grid line will,due to the spherical shape of the earth’s surface, not liein a true northsouth line. The deviation between TrueNorth and grid north at any point due to this curvature,grid declination will never exceed.39. Arbitrarily selectedNorth direction on a drawing or a map, different fromTrue North, usually parallel to or at right angles to someobjects shown, and used for reference in that area only.

Z-rotating prism - See Pechan or Z-rotating prism.

B-31

TM 9-258INDEX

Paragraph PageA

Abbe prism system ............................. 4-5j,4-7d 4-19,4-22Aberrations, elimination ...................... 2-37,2-41 2-42,2-48Absorption, light (See Light)Adapter ............................................... 4-17 4-35Adjustments:

Collimation................................... 5-16 5-11Diopter movement ....................... 5-14 5-10Dioptometer ................................. 5-15 5-11

Aiming circle ....................................... 8-26 8-28Amici prism ......................................... 4-5d,4-7d 4-12, 4-22Astigmatism in lenses ......................... 2-40 2-46Aqueous humor (human eye ............. ) 3-4a 3-3

BBarrel distortion................................. 2 -42h 2-53Bars, optical ........................................ 4-22 4-37Binocular vision................................... 3-7 3-7Binoculars: .

Battery commander’s periscope .. 8-20 8-18General........................................ 8-18 8-16General use ................................. 8-19 8-18

CCaps, objective ................................... 4-20 4-36Cells, lens ........................................... -17 4-35Centering devices ............................... 4-18 4-35Choriod (human eye) .......................... 3-4 3-3Chromatic aberration .......................... 2-39 2-44Coated optics...................................... 4-13 4-31Collimation adjustment ....................... 5-16 5-11Collimator sight ................................... 8-24 8-26Coma .................................................. 2-41 2-48Compound microscope ....................... 5-3 5-1Compton’s theory of light .................... 2.1a(8) 2-2Convergent lens:

Description................................... 2-15 2-20Image formation........................... 2-24 2-32Magnification ............................... 2-24 2-32

Cornea (human eye) ........................... 3-4a 3-3Critical angle

Of various substances ................. 2-17 2-25Total internal reflection .............. 2 -16 2-23

Crystalline lens (human eye) .............. 3-3b 3-2Correction .................................... 2-42i 2-53Curvature:Field............................................. 2-42d 2-50Image, formation.......................... 2-42e 2-51

DDefinition of optical terms(See Glossary.)Devices:

Centering ..................................... 4-18 4-35Focusing ...................................... 4-21 4-36

Diagrams, explanation of symbols .2-4 2-8Diaphragms ........................................ 4-10 4-28Diffraction............................................ 2-436 2-53Diopter: ...............................................

Lens............................................. 10-26 10-1Prism ........................................... 10-2b 10-1

Paragraph PageDiopter movement adjustment ........... 5-14 5-10Dioptometer ........................................ 5-15 5-11Distance

Eye .............................................. 2-34 2-41Interpupillary................................ 3-9f 3-11

Distortion ............................................ 2-42 2-49Divergent lens:

Description .................................. 2-156b 2-23Image formation .......................... 2-25 2-35

Dove prism ......................................... 4-5f 4-13Dynameter, Ramsden......................... 5-24 5-14

EEinstein’s theory of light...................... 2-1a(7) 2-1Electromagnetic spectrum .................. 2-lb (7) 2-1Electromagnetic theory of light

(Maxwell, Boltzmann, and Hertz). 2-1a(5) 2-1Erecting systems ................................ 4-7 4-21Erfle eyepiece..................................... 4-4j 4-9Exit pupil:

Definition ..................................... 2-34 2-41Measurement .............................. 9-3 9-1

Eye, human:Comparison with camera............. 3-2 3-1Defects ........................................ 3-5 3-3Fatigue ........................................ 3-14 3-17Introduction ................................. 3-1 3-1Optical instruments and the eyes:

Accommodation ..................... 3-13 3-17General .................................. 3-12 3-17

Refracting mechanism ................ 3-5 3-3Response mechanism................. 3-6 3-5Structure...................................... 3-3 3-2Tension or fatigue........................ 3-14 3-17Three coats or tunics ................... 3-4 3-3

Eye distance or relief:Definition ..................................... 2-34b 2-41Measurement .............................. 3 9-1

Eyepieces ........................................... 4-4 4-4Eyeshields .......................................... 4-19 4-36Field of view:

FHuman eye ......................................... 3-8 3-8Measurement...................................... 9-5 9-1Optical systems .................................. 2-30,2-31 2-39Field stops .......................................... 4-10 4-28Filters:

Colored........................................ 4-11b 4-29General........................................ 4-11a 4-29Polarizing..................................... 4-11c 4-29

Five-sided prism ................................. 4-5h 4-16Fizeau’s method of measuring

speed of light ............................... 2-2d 2-7Focal length and plane ..................... 2 -22 2730Focusing devices................................ 4-21 4-36Focusing of optical instruments .......... 2-36 2-42Fresnel’s theory of light ...................... 2-1a(4) 2-1

GGalilean telescope .............................. 5-11 5-6

I-1

TM 9-258

Paragraph PageGalilean-type rifle sight ....................... 5-20 5-12Glass, optical ...................................... 4-1 4-1Glossary.............................................. B-2 B-1Greek theory of light ........................... 2-1a(1) 2-1

HHelmet directed subsystem ................ 8-28 8-3Hourglass distortion ............................ 2-42g 2-52Huygenian eyepiece ........................... 4-4h 4-7Huygen’s theory of light ...................... 2-1a(3) 2-1Images:

Brightness.................................... 2-33 2-40Classification ............................... 2-19 2-26Formation by convergent

lens, magnification ................. 2-24 2-32General........................................ 2-18 2-26Reflection by plane mirror ............ 2-20 2-27Transmission by mirror and prism2-212-29

Infrared:Detectors ..................................... 7-4 7-1General........................................ 7-1 7-1Radiation ..................................... 7-2 7-1Thermal ....................................... 7-3 7-1

Instrument lights ................................. 4-12 4-30Intelpupillary distance ......................... 2-35 2-41Iris (human eye) .................................. 3-5b 3-3

KKellner eyepiece ................................. 4-4f 4-5

LLaser rangefinder................................ 8-27 8-29Lasers: ................................................

Gas .............................................. 6-3 6-1General........................................ 6-1 1Optics .......................................... 6-5 6-3Solid state.................................... 6-4 6-2Types........................................... 6-2 6-1

Length, focal ....................................... 2-22 2-30Lens cells............................................ 4-17 4-35Lens, divergent:

Description................................... 2-25 2-35Image formation........................... 2-25 2-35

Lens erecting system.......................... 4-7c 4-21Lenses:

Astigmatism ................................. 2-40 2-46Chromatic aberration, correction . 2-39b 2-45Construction:

Compound.............................. 4-2b 4-2Single ..................................... 4-2a 4-1

Functions of lenses:Collective ..................................... 5-12 5-8Eyepiece...................................... 5-13 5-9Field............................................. 5-12 5-8Ocular .......................................... 5-13 5-9

Magnification, telescope optical system 2-31 2-39Objectives ........................................... 4-3 4-3Resolving power ................................. 2-43 2-53(See also Convergent Lens and

Divergent lens)Lens retaining rings ............................ 4-17 4-35Light:

Color ............................................ 2-lb(6),2-26 2-6,2-37Critical angle ................................ 2-16,2-17 2-23,2-25

Paragraph PageFrequency ................................... 2-3 2-7Image formation .......................... 2-24 2-32Known facts................................. 2-1b 2-1Loss............................................. 2-45 2-55Principle of dispersion ................. 2-27 2-37Selective reflection and absorption 2-28 2-39Separation of "white" light ........... 2-27 2-37Significance of color .................... 2-1b(2) 2-3Speed.......................................... 2-2 2-6Theories ...................................... 2-1 2-1Transmission of (energy)............. 2-1b(1) 2-2Wavelength ............................ 2-3 2-7

Light paths, principle of reversibility2-lb(5)2-6Light rays:

Definition-reflected, emergent,incident, and refracted ........... 2-6,2-11 2-11,2-16

From distant sources ................... 2-1b(4) 2-5General........................................ 2-1b(3) 2-5Properties .................................... 2-1b 2-2Used in diagrams ........................ 2-4 2-8

Lighting............................................... 4-12 4-30M

Magnification:Definition ..................................... 2-23 2-32Telescope .................................. 5-21 5-13

Magnification, eyepiece with variable . 4-4n 4-11Magnifier, simple ................................ 5-2 5-1Maxwell’s theory of light...................... 2-1a(5) 2-1Microscope, compound ...................... 5-3 5-1Military telescopes .............................. 2-30 2-39Millikan’s theory of light ...................... 2-1a(8) 2-2Mirrors, front surface .......................... 4-8 4-25Mirror, plane ....................................... 2-5 2-9

NNewton:

Rings ........................................... 2-44 2-54Theory of light.............................. 2-1a(2) 2-1

Objective caps .................................... 4-20 4-36Optical bars ........................................ 4-22 4-37Optical defects:

Aberrations:Chromatic .................................... 2-39 2-44

General .................................. 2-37 2-42Spherical ................................ 2-38 2-42

Optical glass ....................................... 4-1 4-1Optical instruments: ............................

Accommodation .......................... 3-13 3-17Focusing 2................................... 2-36 2-42General........................................ 3-12 3-17History ......................................... 5-1 5-1Specialized, military:

Collimator sight ...................... 8-24 8-26Reflector (reflex) sight ............ 8-25 8-27

Optical properties of instruments, testing:Determination:

Curvature of image ................ 9-7 9-2Spherical aberration ............... 9-6 9-1

General............................................... 9-1 9-1Magnifying power ............................... 9-4 9-1Measurement:

Field of view ................................ 9-5 9-1Objective aperture .............................. 9-2 9-1

l-2

TM 9-258

Paragraph PageOptical systems:

Field of view................................. 2-32 2-39General........................................ 2-29 2-39Magnification ............................... 2-31 2-39Military telescopes ....................... 2-30 2-39Wedges ....................................... 4-6 4-19

Optics coatedIdentification ................................ 4-15 4-34Introduction, advantages ............. 4-13 4-31Serviceability ............................... 4-16 4-34Theory ......................................... 4-14 4-32

Optics, measurement systems employed:Candlepower and foot candles .... 10-1 10-1Degree system ............................ 10-3 10-1Diopters ....................................... 10-2 10-1Metric system .............................. 10-5 10-4Mil system.................................... 4 10-1

Orthoscopic eyepiece ......................... 4-4k 4-9P

Parallax............................................... 2-46 2-55Pechan prism...................................... 4-5g 4-16Pencils of light .................................... 2-42f 2-52Pentaprism.......................................... 4-5h 4-16Periscopes:

Battery commander’s................... 8-20 8-18Description................................... 5-4 5-2General........................................ 8-13 8-10For self-propelled vehicle ............ 8-1 8-13For tank observation .................... 814 8-12For tanks...................................... 8-16 8-13Infrared ........................................ 8-17 8-15

Plack’s theory of light .......................... 2-1a(6) 2-1Plane mirror, reflection from ............... 2-5 2-9Plane, focal ......................................... 2-22 2-30Plossl eyepiece................................... 4-4m 4-11Porro erecting system......................... 4-7d 4-22Porro prism system............................. 4-5c 4-12Principles of sterovision ...................... 3-9 3-8Prisms................................................. 4-5 4-11Prisms, chromatic aberration, correction 2-39e 2-46Purpose of manual.............................. 1-1 1-1

RRamsden dynameter .......................... 5-24 5-14Rangefinders:

General ........................................ 8-21 8-19Tabulated data .............................. 8-23 8-26Tank (typical coincidence) ............. 8-22 8-21

Rays.................................................... 2-4,2-42f 2-8,2-52Reduction............................................ 2-23 2-32Reflection:

Concave mirror ............................ 2-9 2-13Convex mirror .............................. 2-8 2-13Law .............................................. 2-6 2-11Plane mirror ................................. 2-5 2-9Total internal ................................ 2-16 2-23Types........................................... 2-7 2-11

Reflector (reflex) sight......................... 8-25 8-27Atmospheric................................. 2-13 2-18Refraction:Index............................................ 2-12 2-18Law .............................................. 2-11 2-16Through a glass plate .................. 2-10 2-15Through a triangular glass prism . 2-14 2-20Through lenses ............................ 2-15 2-20

Paragraph PageThrough wedges.......................... 4-6 4-19

Retaining rings ................................... 4-17 4-35Descprition .................................. 4-9 4-26Reticles:Function ...................................... 5-19 5-12

Retina (human eye) ............................ 3-3 3-2Rhombiodal prism .............................. 4-5e 4-13Rifle sight, Galilean-type .................... 5-20 5-12Rings:

Newton’s...................................... 2-44 2-54Retaining ..................................... 4-17 4-35

Roof-angle prism ................................ 4-5d,4-7d(3) 4-12,4-23S

Scope of manual ................................ 1-2 1-3Separators .......................................... 4-17 4-35Sight, collimator .................................. 8-24 8-26Spaces ............................................... 4-17 4-35Spectrum, electromagnetic ................. 2-1b(2) 2-3Spherical aberration:

Elimination................................... 2-38e,f 2-43,2-44General........................................ 2-38 2-42

Stereovision, principles ...................... 3-9 3-8Stops .................................................. 4-10 4-28Sunshades ......................................... 4-20 4-36Symbols.............................................. 2-4 2-8Symmetrical eyepiece ........................ 4-4I 4-9

TTabulated data ................................... 8-23 8-26Telescope design, optical factors:

Factors determining:Eye relief.................................. 5-27 5-16Field of view............................. 5-26 5-15

Light transmission or illumination of image 5-28 5-17Magnification or power of telescope 5-21 5-13

Night glasses ...................................... 5-29 5-17Optical glass used in design

of military instruments ................. 5-31 5-18Pupil:

Entrance ...................................... 5-22 5-13Exit .............................................. 5-23 5-1

Ramsden dynameter .......................... 5-24 5-14Resolving power of optical systems ... 5-30 5-17True and apparent fields of telescope 6-25 5-14Telescopes:

ArticulatedFor self-propelled vehicle......... 8-7 8-4For tank.................................... 8-6 8-4General .................................... 8-5 8-3

Astronomical:Magnification ............................... 5-7 5-4Reflecting .................................... 5-6 5-3Refracting .................................... 5-5 5-2

Elbow:For howitzer (towed &

self-propelled) .......................... 8-9a,8-9c,8-9d 8-5,8-7For sight units.............................. 8-9b 8-6General........................................ 8-8 8-5Typical ......................................... 8-9 8-5

General............................................... 8-1 8-1Military ................................................ 2-30 2-39Observation ........................................ 8-4 8-2Panoramic:

For self-propelled howitzer .......... 8-12 8-9For towed howitzer ...................... 8-11 8-9General........................................ 8-10 8-8

1-3

TM 9-258

Paragraph ...................................PageRifle ............................................. 8-2 8-1Sniper’s sighting device ............... 8-3 8-1Observation ................................. 8-4 8-2

Terrestrisail:Galilean ....................................... 5-11 5-6General........................................ 5-8 5-4Lens erecting systems ................. 5-9 5-4Prism erecting systems ............... 5-10 5-6

Telescopic sights, functions:Advantages.................................. 5-18 5-12Definition...................................... 5-17 5-12Functions of reticles..................... 5-19 5-12Galilean-type rifle sight ................ 5-20 5-12

Testing optical properties of instruments 9-1 thru 9-1,9-29-7

Triple mirror prism............................... 4-5i 4-18V

View, field ........................................... 3-8 3-8

Paragraph................................... PageVision:

Binocular ..................................... 3-7 3-7Defects of (human eye) ............... 3-10 3-14Steroacuity .................................. 3-9k 3-14Stereovision................................. 3-9 3-8

Visual limitations................................. 3-11 3-15

WWave energy transmission ................. 2-1b 2-2Wavelength and frequency ................. 2-3 2-7Wavelength, units of measurement .... 2-1b 2-2Wedges, optical .................................. 4-6 4-19

YYoung’s theory of light ........................ 2-1a(4) 2-1

ZZ prism ............................................... 4-5g 4-16

I-4

TM 9-258

By Order of the Secretary of the Army:

BERNARD W. ROGERSGeneral, United States Army

Chief of StaffOfficial:

J. C. PENNINGTONBrigadier General, United States Army

The Adjutant General

DISTRIBUTION:

To be distributed in accordance with DA Form 12-41, Direct/General Support maintenance for Rangefinder, Binocular,Periscope, and Telescope.

* U.S. GOVERNMENT PRINTING OFFICE : 1990 0 - 261-888 (22709)

PIN: 026334-000

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DEPARTMENT OF THE ARMY TECHNICAL MANUAL · tm 9-258 technical manual headquarters department of the army no. 9-258 washington, dc, 5 december 1977 elementary optics and application